Accessing the free expansion of a crystalline colloidal drop by optical experiments.

We study poly-crystalline spherical drops of an aqueous suspension of highly charged colloidal spheres exposed to a colloid-free aqueous environment. Crystal contours were obtained from standard optical imaging. The crystal spheres first expand to nearly four times their initial volume before slowly shrinking due to dilution-induced melting. Exploiting coherent multiple-scattering by (110) Bragg reflecting crystals, time-dependent density profiles were recorded within the drop interior. These show a continuously flattening radial density gradient and a decreasing central density. Expansion curves and density profiles are qualitatively consistent with theoretical expectations based on dynamical density functional theory for the expansion of a spherical crystallite made of charged Brownian spheres. We anticipate that our study opens novel experimental access to density determination in turbid crystals.

Magnetic response of CoFe2O4 nanoparticles confined in a PNIPAM microgel network.

The paper addresses coupling of magnetic nanoparticles (MNPs) with the polymer matrix of temperature-sensitive microgels and their response to magnetic fields. Therefore, CoFe2O4@CA (CA = citric acid) NPs are embedded within N-isopropylacrylamid (NIPAM) based microgels. The volume phase transition (VPT) of the magnetic microgels and the respective pure microgels is studied by dynamic light scattering and electrophoretic mobility measurements. The interaction between MNPs and microgel network is studied via magnetometry and AC-susceptometry using a superconducting quantum interference device (SQUID). The data show a significant change of the magnetic properties by crossing the VPT temperature (VPTT). The change is related to the increased confinement of the MNP due to the shrinking of the microgels. Modifying the microgel with hydrophobic allyl mercaptan (AM) affects the swelling ability and the magnetic response, i.e. the coupling of MNPs with the polymer matrix. Modeling the AC-susceptibility data results in an effective size distribution. This distribution represents the varying degree of constraint in MNP rotation and motion by the microgel network. These findings help to understand the interaction between MNPs and the microgel matrix to design multi responsive systems with tunable particle matrix coupling strength for future applications.

Are men who have sex with men in Europe protected from hepatitis B?

Hepatitis B vaccination is recommended for men who have sex with men (MSM) in many countries, but information on vaccine coverage is scarce. We studied hepatitis B vaccination programmes and coverage among MSM in Europe to guide prevention. From a large (N = 174 209) pan-European MSM survey (EMIS-2010), we used data on self-reported hepatitis B vaccination, age, education, settlement size and disclosure of the same-sex sexual orientation ('outness'). We excluded participants with a history of hepatitis B. In multilevel (participants, countries) logistic regression models, we calculated adjusted odds ratios (aOR) with 95% confidence intervals (95% CI). We analysed data of 163 987 MSM in 38 European countries: 38.3% were 'out' to all or almost all, 56.4% reported vaccination against hepatitis B and 65.5% lived in countries with free recommended hepatitis B vaccination for MSM. In the final model the odds for being vaccinated increased with outness ('out to all or almost all': aOR 1.76, 95% CI 1.70-1.83 vs. 'out to no one') and with living in countries, where hepatitis B vaccination was recommended and free-of-charge for MSM (aOR 2.21, 95% CI 1.47-3.32 vs. 'no or unclear recommendation'). To increase hepatitis B vaccination coverage among MSM, implementation of MSM-specific recommendations and improvement of the societal climate for MSM is needed.

Factors related to condomless anal intercourse between men who have sex with men: results from a European bio-behavioural survey.

BACKGROUND: Relationship status is an important factor associated with condomless anal intercourse (CAI) amongst men who have sex with men (MSM). METHODS: A multi-centre bio-behavioural survey with MSM was conducted in 13 European cities (n = 4901) exploring factors associated with CAI via bivariate and multivariate multilevel logistic regression analyses. RESULTS: Likelihood of CAI with casual partners was associated with being 'out' to a majority (AOR = 1.19;95% CI 1,1.42); knowing their HIV status (AOR = 1.86; 95% CI 1.25,2.76); using substances (1-2 AOR = 1.39; 95% CI 1.16,1.63, 2+ AOR = 1.81; 95% CI 1.35,2.42); being older (AOR = 0.98; 95% CI 0.97,0.99); successful sero-communication (AOR = 0.79; 95% CI 0.67,0.94); and, not having a recent HIV test (AOR = 0.78; 95% CI 0.66,0.92). CAI with steady partners was associated with successful sero-communication (AOR = 2.72; 95% CI 2.72,3.66); not having a recent HIV test (AOR = 1.26; 95% CI 1.09,1.46), and; being older (AOR = 0.99; 95% CI 0.98,0.99). CONCLUSIONS: Understandings of partner type and/or relationship status in relation to CAI amongst MSM can potentially play an important role in the development of culturally appropriate HIV/STI prevention and risk-reduction efforts targeting at-risk MSM. Our results speak to the need to consider segmented and tailored public health and health promotion initiatives for MSM with differing CAI behaviours and relationship profiles.

A comparison of the network structure and inner dynamics of homogeneously and heterogeneously crosslinked PNIPAM microgels with high crosslinker content.

Poly(N-isopropylacrylamide) microgel particles were prepared via a "classical" surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.

Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach.

The preparation of poly(N-isopropylacrylamide) microgels via classical precipitation polymerization (batch method) and a continuous monomer feeding approach (feeding method) leads to different internal crosslinker distributions, i.e., from core-shell-like to a more homogeneous one. The internal structure and dynamics of these microgels with low and medium crosslinker concentrations are studied with dynamic light scattering and small-angle neutron scattering in a wide q-range below and above the volume phase transition temperature. The influence of the preparation method, and crosslinker and initiator concentration on the internal structure of the microgels is investigated. In contrast to the classical conception where polymer microgels possess a core-shell structure with the averaged internal polymer density distribution within the core part, a detailed view of the internal inhomogeneities of the PNIPAM microgels and the presence of internal domains even above the volume phase transition temperature, when polymer microgels are in the deswollen state, are presented. The correlation between initiator concentration and the size of internal domains that appear inside the microgel with temperature increase is demonstrated. Moreover, the influence of internal inhomogeneities on the dynamics of the batch- and feeding-microgels studied with neutron spin-echo spectroscopy is reported.

Monitoring the HIV continuum of care in key populations across Europe and Central Asia.

OBJECTIVES: The aim of the study was to measure and compare national continuum of HIV care estimates in Europe and Central Asia in three key subpopulations: men who have sex with men (MSM), people who inject drugs (PWID) and migrants. METHODS: Responses to a 2016 European Centre for Disease Prevention and Control (ECDC) survey of 55 European and Central Asian countries were used to describe continuums of HIV care for the subpopulations. Data were analysed using three frameworks: Joint United Nations Programme on HIV/AIDS (UNAIDS) 90-90-90 targets; breakpoint analysis identifying reductions between adjacent continuum stages; quadrant analysis categorizing countries using 90% cut-offs for continuum stages. RESULTS: Overall, 29 of 48 countries reported national data for all HIV continuum stages (numbers living with HIV, diagnosed, receiving treatment and virally suppressed). Six countries reported all stages for MSM, seven for PWID and two for migrants. Thirty-one countries did not report data for MSM (34 for PWID and 41 for migrants). In countries that provided key-population data, overall, 63%, 40% and 41% of MSM, PWID and migrants living with HIV were virally suppressed, respectively (compared with 68%, 65% and 68% nationally, for countries reporting key-population data). Variation was observed between countries, with higher outcomes in subpopulations in Western Europe compared with Eastern Europe and Central Asia. CONCLUSIONS: Few reporting countries can produce the continuum of HIV care for the three key populations. Where data are available, differences exist in outcomes between the general and key populations. While MSM broadly mirror national outcomes (in the West), PWID and migrants experience poorer treatment and viral suppression. Countries must develop continuum measures for key populations to identify and address inequalities.

Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels.

The present study addresses the multiresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this so-called cononsolvency effect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specific surface details, as well as the sample preparation. Thereby, the microgels are deposited on two different kinds of surfaces: on gold surface and on polycation (PAH) coating which show different interactions with the microgels in terms of electrostatic interaction and wettability. In addition, the microgels were deposited from different solvent mixtures. This influences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T > VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

A cluster of invasive meningococcal disease in young men who have sex with men in Berlin, October 2012 to May 2013.

Between October 2012 and May 2013, five cases of invasive meningococcal disease in young men who have sex with men (MSM) living in Berlin were notified to local health authorities in Germany. Three of the five cases died. All were caused by serogroup C variants with the finetype P1.5-1,10-8:F3-6. Awareness was increased through the use of community networks; an extension of the existing vaccination recommendation to all MSM is currently being considered.

[Deficits in STI diagnosis for men who have sex with men (MSM) in German-speaking cities: results of the European MSM Internet Survey (EMIS)]. / Defizite in den Angeboten zur STI-Diagnostik für Männer mit gleichgeschlechtlichen Sexualkontakten in deutschsprachigen Städten : Ergebnisse des Europäischen MSM-Internet-Surveys (EMIS).

BACKGROUND: Sexually transmitted infections (STIs) such as anal/genital warts, syphilis, and genital/rectal gonorrheal/chlamydial infections compromise the health of men who have sex with men (MSM), and increase the per-contact risk of HIV infection. Early detection of asymptomatic STIs requires regular screening including physical examinations and collection of clinical specimens that allow for the detection of infections at sites common to men's same-sex practices. METHODS: From June to August 2010, the European MSM Internet Survey (EMIS) recruited 174,209 MSM from 38 European countries to an anonymous online questionnaire in 25 languages. As sexual health care for MSM in most countries is organized locally, we chose cities for comparison. Multivariable regression models were used to compare accessibility of services and applied diagnostic procedures across 1 Dutch, 1 Swiss, 1 Austrian, 3 English, and 19 German cities (N = 29,962). RESULTS: The proportion of respondents tested for STIs in the last 12 months in the absence of symptoms ranged from 13 % in Magdeburg to 48.0 % in Amsterdam. At a city level, low STI screening correlated with inaccessible services (R(2) = 0.72). At an individual level, anal/penile inspection and anal swabbing was most common in English cities and in Amsterdam. Compared to London, MSM in German-speaking cities had an adjusted odds ratio (AOR) of 0.06-0.20 for anal/penile inspection, and of 0.05-0.17 for anal swabbing (p < 0.001). CONCLUSIONS: Anal/genital warts and rectal infections are likely to be profoundly underdiagnosed among MSM in all German-speaking cities. This has implications for the sexual health of MSM, for HIV prevention, and for comparing European surveillance data. There is an urgent need to implement or improve sexual health care tailored to MSM at risk for STIs.

[Epidemiological developments of selected sexually transmitted infections in Germany]. / Epidemiologische Entwicklung bei ausgewählten sexuell übertragbaren Infektionen (STI) in Deutschland.

The number of people living with HIV infection has been increasing since the mid 1990s and is expected to rise further in the coming years. The HIV epidemic in Germany is still most affected by developments in the group of men who have sex with men (MSM). In this group, the number of newly diagnosed HIV infections has increased in recent years especially in large cities. Despite increased efforts to motivate HIV-infected people, who were not previously diagnosed, to be tested as early as possible and to seek medical treatment, the number of undiagnosed HIV-infected persons has increased. There are more people infected with HIV than those who have been tested positive for HIV and subsequently receive antiretroviral treatment. However, early testing and treatment alone are not sufficient to effectively contain the infection. Increased efforts are required to more effectively prevent new HIV infections by combining all the available options. In Germany as in all other developed countries, a stronger increase in the number of syphilis infections among MSM is reported, which is primarily due to a higher willingness to risk unprotected contacts, whereby the risk of HIV infection is also increased. The public prevention messages available for HIV are only partially effective against syphilis. More frequent examinations and optimized therapy management are necessary in addition to the use of condoms to prevent the spread of syphilis, gonorrhea, and Chlamydia trachomatis. Sustainable containment of new HIV infections must, therefore, be accompanied by both containment of sexually transmitted infections (STI) and use of public prevention messages for HIV/STI.

Late presentation for HIV diagnosis and care in Germany.

OBJECTIVES: Antiretroviral therapy reduces mortality and morbidity in HIV-infected individuals most markedly when initiated early, before advanced immunodeficiency has developed. Late presentation for diagnosis and care remains a significant challenge. To guide public health interventions effectively it is crucial to describe the factors associated with late presentation. METHODS: Case surveillance data for all individuals newly diagnosed with HIV infection in Germany in the years 2001-2010 and data for the years 1999-2010 from the German Clinical Surveillance of HIV Disease (ClinSurv) cohort study, a large multicentre observational study, were analysed. Factors associated with late presentation (CD4 count < 350 cells/µL or clinical AIDS) were assessed using descriptive statistics and multivariable logistic regression methods. RESULTS: Among 22 925 eligible patients in the national surveillance database, 49.5% were late presenters for HIV diagnosis. Among 6897 treatment-naïve patients in the ClinSurv cohort, 58.1% were late presenters for care. Late presenters for care were older (median 42 vs. 39 years for early presenters), more often heterosexuals from low-prevalence countries (18.1% vs. 15.5%, respectively) and more often migrants (18.2% vs. 9.7%, respectively; all P < 0.005). The probability of late presentation was >65% throughout the observation period in migrants. The probability of late presentation for care clearly decreased in men who have sex with men (MSM) from 60% in 1999 to 45% in 2010. CONCLUSIONS: In Germany, the numbers of late presenters for HIV diagnosis and care remain high. The probability of late presentation for HIV diagnosis seems to be particularly high for migrants. These results argue in favour of targeted test promotion rather than opt-out screening. Late presentation for care seems to be an additional problem after HIV diagnosis.

Syphilis on the rise again in Germany--results from surveillance data for 2011.

In Germany, syphilis is notified anonymously. In 2011, 3,698 cases (incidence 4.5/100,000 inhabitants) were notified, an increase of 22% over 2010. The increase was higher in men (23%) than women (13%) and 94% of the cases were male. Information on the possible way of transmission was available for 72% of cases. Of these, 84% were men who have sex with men, who seem to play a major role in the renewed increase in syphilis cases.

Bridging the gap between two different scaling laws for structuring of liquids under geometrical confinement.

Structural forces are a phenomena obtained in liquids of one-component (e.g. for organic solvents) and two-components (colloidal dispersions), alike. So far, those two systems were discussed separately, using two different scaling laws. In this review article, an attempt is made to bridge the gap between both scaling laws by defining the scaling limit for two-component systems. Colloidal probe atomic force microscopy (CP-AFM) is used to measure structural forces in suspensions of silica nanoparticles (NPs) of three different sizes. In these two-component systems (solid NPs suspended in water), oscillatory behaviour can be obtained in the force vs. separation profiles. The wavelength λ is larger than the actual particle diameter d and rather depends on the particles' volume fraction ϕ following the inverse cubic root law λ∝ϕ-13. It is shown that the real particle diameter d can be determined by a gedankenexperiment by extrapolating the fitted wavelength λ from the structural force measurements at a specific particle concentration to a particle volume fraction ϕ of 52% - the packing factor for simple cubic packing - using the well-known inverse cubic root scaling law. This extrapolation can be interpreted as a transition from a two-component system towards a one-component-like problem. In this case, particles are in contact and the wavelength λ is equal to the particle diameter d, λ = d as for one-component systems. The determined diameters d of the different silica nanoparticles agree well with independent measurements using transmission electron microscopy (TEM), validating the used approach. The proposed method can be extended to numerous dispersions of spherical nano-sized objects, for which structural forces can be measured.

Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based Microgels of Different Cross-linker Distributions.

The aim of this study is to tailor the inner structure of positively charged poly-( N-isopropylacrylamid- co-allylamine) (P(NIPAM- co-AA)) microgels for a better control of the distribution of negatively charged magnetic cobaltferrite (CoFe2O4@CA) nanoparticles (MNPs) within the microgels. Therefore, two different strategies are followed for the microgel synthesis: the (one pot) batch method which leads to a higher cross-linker density in the microgel core and the feeding method which compensates different reaction kinetics of the cross-linker and the monomers. The latter one is expected to result in a homogeneous cross-linker distribution. Information about the cross-linker distribution is indirectly gained by measuring the elastic modulus via indentation experiments with an atomic force microscope. While the batch method results in a higher elastic modulus in the center of the microgel indicating a core/shell structure, the feeding method leads to a constant elastic modulus over the whole microgel. The loading with MNPs and their distribution are studied with transmission electron microscopy (TEM). The TEM images show a large difference in the MNP distribution which is correlated to the cross-linker distribution of both types of microgels. The batch method microgel has a low MNP concentration in the core. The feeding method microgel shows a much more homogeneous distribution of MNPs across the microgel. The latter one also shows a stronger charge reversal which is a hint for a higher loading of the feeding method microgel. Dynamic light scattering and electrophoretic mobility measurements demonstrate that for both types of microgels, the temperature sensitivity is preserved after loading with MNPs.

A large proportion of people who inject drugs are susceptible to hepatitis B: Results from a bio-behavioural study in eight German cities.

BACKGROUND: People who inject drugs (PWID) are at high risk of hepatitis B virus (HBV) infection by sharing needles and drug use paraphernalia. In Germany, no routine surveillance of HBV prevalence and vaccination coverage among PWID exists. METHODS: Socio-demographic and behavioural data were collected between 2011 and 2014 through face-to-face interviews, during a bio-behavioural survey of PWID recruited in eight German cities. Dried blood spots (DBS) prepared with capillary blood were tested for HBV markers. Factors associated with past/current HBV infection and vaccination status were analysed by univariable and multivariable analysis using logistic regression. The validity of self-reported HBV infection and vaccination status was analysed by comparison to the laboratory results. RESULTS: Among 2077 participants, the prevalence of current HBV infection was 1.1%, of past HBV infection was 24%, and of vaccine-induced HBV antibodies was 32%. No detectable HBV antibodies were found in 43%. HBV infection status was significantly associated with study city, age, years of injecting, use of stimulants, migration status, and homelessness; HBV vaccination status was significantly associated with study city, age, and level of education. Correct infection status was reported by 71% and correct vaccination status by 45%. CONCLUSIONS: HBV seroprevalence among PWID was about five times higher than in the general population in Germany, confirming PWID as an important risk group. Targeted information campaigns on HBV and HBV prevention for PWID and professionals in contact with PWID need to be intensified. Routinely offered HBV vaccination during imprisonment and opioid substitution therapy would likely improve vaccination rates among PWID.

Poly(N-isopropylacrylamide) Microgels under Alcoholic Intoxication: When a LCST Polymer Shows Swelling with Increasing Temperature.

Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.

German-Austrian recommendations for HIV-therapy in pregnancy and in HIV-exposed newborn - update 2005.

In Germany during the last years about 200-250 HIV infected pregnant women delivered a baby each year, a number that is currently increasing. To determine the HIV-status early in pregnancy voluntary HIV-testing of all pregnant women is recommended in Germany and Austria as part of prenatal care. In those cases, where HIV infection was known during pregnancy, since 1995 the rate of vertical transmission of HIV was reduced to 1-2%. - This low transmission rate has been achieved by the combination of anti-retroviral therapy of pregnant women, caesarean section scheduled before onset of labour, anti-retroviral post exposition prophylaxis in the newborn and refraining from breast-feeding by the HIV infected mother. To keep pace with new results in research, approval of new anti-retroviral drugs and changes in the general treatment recommendations for HIV infected adults, in 1998, 2001 and 2003 an interdisciplinary consensus meeting was held. Gynaecologists, infectious disease specialists, paediatricians, pharmacologists, virologists and members of the German AIDS Hilfe (NGO) were participating in this conference to update the prevention strategies. A third update became necessary in 2005. The updating process was started in January 2005 and was terminated in September 2005. The guidelines provide new recommendations on the indication and the starting point for therapy in pregnancies without complications, drugs and drug combinations to be used preferably in these pregnancies and updated information on adverse effects of anti-retroviral drugs. Also the procedures for different scenarios and risk constellations in pregnancy have been specified again. With these current guidelines in Germany and Austria the low rate of vertical HIV-transmission should be further maintained.

Lymphogranuloma venereum emerging in men who have sex with men in Germany.

A resurgence of lymphogranuloma venereum (LGV) has been observed in several European countries. LGV is not a mandatorily notifiable disease in Germany. Reports of LGV cases have actively been collected by the Robert Koch-Institut since 2004 to describe the outbreak and estimate the extent of the LGV problem in Germany. Updates on the LGV outbreak were published in the German national epidemiological bulletin. Physicians were asked to send their samples to a laboratory for genotyping. A possible case was defined as a person with symptoms of proctitis and/or inguinal lymph node swelling and a positive chlamydia serology. A probable case had in addition a positive chlamydia rectal or urinary PCR test. A case was confirmed if the genotype L1-L3 was identified based on sequence analysis of omp1 gene sequences. Since 2003, LGV has been reported in 78 male patients in Germany. Of these, 61 patients were confirmed as genotype L2. Fifty eight out of 78 patients (74%) are known to be men who have sex with men (MSM). Fifty five patients (71%) had rectal symptoms and 49 (63%) knew they were HIV positive. Sixty two (79%) of the patients were residents of Berlin or Hamburg. LGV has emerged in MSM in Germany at the same time as in other European countries. It is thought that LGV may become endemic in the MSM community in German metropolitan areas, because the number of reported patients with LGV continues to increase. The increase in the number of LGV cases and the high HIV prevalence in LGV patients are of great public health concern. Clinicians and MSM may not be sufficiently aware of the disease, and existing efforts to promote awareness and prevention of sexually transmitted infections and HIV need to be strengthened.

HIV incidence increasing in MSM in Germany: factors influencing infection dynamics.

After an initial peak in the mid-1980s, HIV incidence in men who have sex with men (MSM) declined in most western industrialised countries and then levelled off during the 1990s. Since the late 1990s, increasing numbers of newly diagnosed HIV infections in MSM have been observed in the majority of countries with large and visible MSM communities. Based on a review of national and international behavioural surveillance studies of MSM and national HIV surveillance data, we propose a model for the HIV epidemic in MSM in Germany. The model includes aspects such as individuals' increasing numbers of sexual partners and increasing frequency of unprotected anal intercourse, conditional condom use based on real or perceived HIV status of sexual partners (HIV 'serosorting') and sexual role assignments (insertive versus receptive based on HIV status (HIV 'seropositioning'), selection of partners and formation of sexual networks through seeking sexual partners on the internet, the introduction of HAART and changing HAART treatment strategies. All these aspects have been shown or are suspected to increase or decrease HIV transmission risk in MSM. We conclude that increasing HIV incidence in MSM in recent years has been fuelled by a spread of HIV in high-risk sexual networks with an increasing proportion of infections transmitted during highly infective early HIV infection, acquired mostly from casual sexual partners.