Olfactory dysfunction and COVID-19.

Here, we provide an overview of olfactory dysfunction associated with COVID-19. We provide background regarding the organization and function of the peripheral olfactory system. A review of the relevant literature on anosmia and parosmia due to infection with SARS-CoV-2, the virus causing COVID-19, is provided. Specific attention is focused on possible mechanisms by which the virus may interact with and damage the cell populations of peripheral olfactory system. Evidence from human studies as well as animal models is considered. Finally, we discuss current recommendations for evaluation and management of patients with persistent post-COVID olfactory dysfunction, as well as possible future research directions.

Identifying Treatments for Taste and Smell Disorders: Gaps and Opportunities.

The chemical senses of taste and smell play a vital role in conveying information about ourselves and our environment. Tastes and smells can warn against danger and also contribute to the daily enjoyment of food, friends and family, and our surroundings. Over 12% of the US population is estimated to experience taste and smell (chemosensory) dysfunction. Yet, despite this high prevalence, long-term, effective treatments for these disorders have been largely elusive. Clinical successes in other sensory systems, including hearing and vision, have led to new hope for developments in the treatment of chemosensory disorders. To accelerate cures, we convened the "Identifying Treatments for Taste and Smell Disorders" conference, bringing together basic and translational sensory scientists, health care professionals, and patients to identify gaps in our current understanding of chemosensory dysfunction and next steps in a broad-based research strategy. Their suggestions for high-yield next steps were focused in 3 areas: increasing awareness and research capacity (e.g., patient advocacy), developing and enhancing clinical measures of taste and smell, and supporting new avenues of research into cellular and therapeutic approaches (e.g., developing human chemosensory cell lines, stem cells, and gene therapy approaches). These long-term strategies led to specific suggestions for immediate research priorities that focus on expanding our understanding of specific responses of chemosensory cells and developing valuable assays to identify and document cell development, regeneration, and function. Addressing these high-priority areas should accelerate the development of novel and effective treatments for taste and smell disorders.

Contribution of Polycomb group proteins to olfactory basal stem cell self-renewal in a novel c-KIT+ culture model and in vivo.

Olfactory epithelium (OE) has a lifelong capacity for neurogenesis due to the presence of basal stem cells. Despite the ability to generate short-term cultures, the successful in vitro expansion of purified stem cells from adult OE has not been reported. We sought to establish expansion-competent OE stem cell cultures to facilitate further study of the mechanisms and cell populations important in OE renewal. Successful cultures were prepared using adult mouse basal cells selected for expression of c-KIT. We show that c-KIT signaling regulates self-renewal capacity and prevents neurodifferentiation in culture. Inhibition of TGFß family signaling, a known negative regulator of embryonic basal cells, is also necessary for maintenance of the proliferative, undifferentiated state in vitro Characterizing successful cultures, we identified expression of BMI1 and other Polycomb proteins not previously identified in olfactory basal cells but known to be essential for self-renewal in other stem cell populations. Inducible fate mapping demonstrates that BMI1 is expressed in vivo by multipotent OE progenitors, validating our culture model. These findings provide mechanistic insights into the renewal and potency of olfactory stem cells.

Long-term olfactory loss post-COVID-19: Pathobiology and potential therapeutic strategies.

An acute loss of smell emerged as a striking symptom present in roughly half of the people infected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus in the early phases of the COVID-19 pandemic. In most COVID-19 patients, olfaction recovers over the course of a few weeks. However, a lasting partial or complete loss of smell, often associated with distorted olfactory perceptions termed parosmia, has emerged as a widespread problem impacting at least 5%-10% of those who experience anosmia due to COVID-19. Our inability to offer effective therapies to this hyposmic or anosmic population, comprising millions of patients, highlights an enormous unmet need for the medical system. Here, we summarize the current understanding of the pathobiology causing acute olfactory loss due to SARS-CoV-2 infection, focusing on how the virus interacts with the peripheral olfactory system, a major site of viral infection. We also explore the problem of long-COVID olfactory dysfunction, which may accompany other persistent systemic disorders collectively termed postacute sequelae of COVID-19. Specifically, we discuss an emerging model focused on unresolved immune cell activity driving ongoing dysfunction. Finally, we review current and future therapeutic approaches aimed at restoring olfactory function.

Patient-derived organoid model of olfactory ensheathing cell tumor.

KEY POINTS: We developed a culture model of a human olfactory ensheathing cell tumor. Cultured organoids resemble normal ensheathing cells. Assays suggest that this model provides a tool for studying the roles of these glial cells in the maintenance of the peripheral olfactory system.

Safety and Feasibility of Steroid-Eluting Stent as a Bolster in Endoscopic Anterior Skull Base Reconstruction.

BACKGROUND: With a rising incidence of cerebrospinal fluid (CSF) leaks, endoscopic endonasal CSF leak repair is increasingly performed. Current approaches utilize a variety of materials including free mucosal grafts and vascularized flaps, but post-op leaks continue to be reported. Steroid-eluting bioabsorbable stents (SES) are used during functional endoscopic sinus surgery for chronic rhinosinusitis to reduce inflammation and scarring while maintaining patency of sinus ostia. OBJECTIVE: The aim of this study is to assess the feasibility of SES as a graft/flap bolster for endoscopic endonasal CSF leak repair. METHODS: This is a retrospective review of patients undergoing endoscopic endonasal CSF leak repair with SES placed as part of the bolster technique at a tertiary care center between January 2019 and May 2022. Age, sex, BMI, comorbid idiopathic intracranial hypertension, pathology, location of CSF leak, intraoperative CSF leak flow, reconstruction type, and presence of post-op CSF leak were recorded. RESULTS: Twelve patients (mean age 52, median BMI 30.9, 58% female) had SES placement as part of the bolster technique. The most common pathology was meningoencephalocele (75%). Reconstruction was performed with either a free mucosal graft (6), or a flap (6). No post-op CSF leaks occurred at a reconstruction site with a stent, and no known complications were reported. All sinusotomies were patent at the last follow-up visit. CONCLUSIONS: SES placement as an adjunct to graft and/or flap bolster appears to be safe and feasible during anterior skull base reconstruction and CSF leak repair providing longer term structural support and preserving sinus drainage patency.

Olfactory neuroblastoma mimics molecular heterogeneity and lineage trajectories of small-cell lung cancer.

The olfactory epithelium undergoes neuronal regeneration from basal stem cells and is susceptible to olfactory neuroblastoma (ONB), a rare tumor of unclear origins. Employing alterations in Rb1/Trp53/Myc (RPM), we establish a genetically engineered mouse model of high-grade metastatic ONB exhibiting a NEUROD1+ immature neuronal phenotype. We demonstrate that globose basal cells (GBCs) are a permissive cell of origin for ONB and that ONBs exhibit cell fate heterogeneity that mimics normal GBC developmental trajectories. ASCL1 loss in RPM ONB leads to emergence of non-neuronal histopathologies, including a POU2F3+ microvillar-like state. Similar to small-cell lung cancer (SCLC), mouse and human ONBs exhibit mutually exclusive NEUROD1 and POU2F3-like states, an immune-cold tumor microenvironment, intratumoral cell fate heterogeneity comprising neuronal and non-neuronal lineages, and cell fate plasticity-evidenced by barcode-based lineage tracing and single-cell transcriptomics. Collectively, our findings highlight conserved similarities between ONB and neuroendocrine tumors with significant implications for ONB classification and treatment.

Preparation of Human Olfactory Epithelial Biopsies for Downstream Analysis.

The olfactory mucosa, lining a portion of the nasal cavity, houses the primary olfactory sensory neurons responsible for odor transduction, along with supporting cell populations. Tremendous advances have come from studying the peripheral olfactory system in animal models, especially the mouse. However, acquired human olfactory disorders lack effective therapies, and many of these conditions involve pathology in the olfactory mucosa. Thus, the ability to obtain human olfactory biopsy samples from subjects with olfactory dysfunction, or controls, may be of value. Here, we describe established techniques for collecting olfactory tissue from human subjects and preparing samples for downstream assays such as immunohistochemistry, flow cytometry, single-cell RNA-sequencing, or chromatin studies.

Deconstructing Olfactory Epithelium Developmental Pathways in Olfactory Neuroblastoma.

Olfactory neuroblastoma is a rare tumor arising from the olfactory cleft region of the nasal cavity. Because of the low incidence of this tumor, as well as an absence of established cell lines and murine models, understanding the mechanisms driving olfactory neuroblastoma pathobiology has been challenging. Here, we sought to apply advances from research on the human olfactory epithelial neurogenic niche, along with new biocomputational approaches, to better understand the cellular and molecular factors in low- and high-grade olfactory neuroblastoma and how specific transcriptomic markers may predict prognosis. We analyzed a total of 19 olfactory neuroblastoma samples with available bulk RNA-sequencing and survival data, along with 10 samples from normal olfactory epithelium. A bulk RNA-sequencing deconvolution model identified a significant increase in globose basal cell (GBC) and CD8 T-cell identities in high-grade tumors (GBC from ∼0% to 8%, CD8 T cell from 0.7% to 2.2%), and significant decreases in mature neuronal, Bowman's gland, and olfactory ensheathing programs, in high-grade tumors (mature neuronal from 3.7% to ∼0%, Bowman's gland from 18.6% to 10.5%, olfactory ensheathing from 3.4% to 1.1%). Trajectory analysis identified potential regulatory pathways in proliferative olfactory neuroblastoma cells, including PRC2, which was validated by immunofluorescence staining. Survival analysis guided by gene expression in bulk RNA-sequencing data identified favorable prognostic markers such as SOX9, S100B, and PLP1 expression. Significance: Our analyses provide a basis for additional research on olfactory neuroblastoma management, as well as identification of potential new prognostic markers.

Polycomb repressive complex 2 regulates basal cell fate during adult olfactory neurogenesis.

Adult neurogenesis occurs in the mammalian olfactory epithelium to maintain populations of neurons that are vulnerable to injury yet essential for olfaction. Multipotent olfactory basal stem cells are activated by damage, although mechanisms regulating lineage decisions are not understood. Using mouse lesion models, we focused on defining the role of Polycomb repressive complexes (PRCs) in olfactory neurogenesis. PRC2 has a well-established role in developing tissues, orchestrating transcriptional programs via chromatin modification. PRC2 proteins are expressed in olfactory globose basal cells (GBCs) and nascent neurons. Conditional PRC2 loss perturbs lesion-induced neuron production, accompanied by altered histone modifications and misexpression of lineage-specific transcription factors in GBCs. De-repression of Sox9 in PRC2-mutant GBCs is accompanied by increased Bowman's gland production, defining an unrecognized role for PRC2 in regulating gland versus neuron cell fate. Our findings support a model for PRC2-dependent mechanisms promoting sensory neuronal differentiation in an adult neurogenic niche.

Characterizing Olfactory Dysfunction in Patients with Unilateral Cleft Lip Nasal Deformities.

Background: Unilateral cleft lip nasal deformity (uCLND) is associated with olfactory dysfunction, but the underlying etiology remains poorly understood. Objective: To investigate the etiology of uCLND-associated olfactory dysfunction using clinical, computational, and histologic assessments. Methods: Inclusion criteria: uCLND patients >16 years undergoing septorhinoplasty. Exclusion criteria: prior septoplasty or rhinoplasty, pregnancy, sinusitis. Measured outcomes: patient-reported scores, rhinomanometry, smell identification and threshold tests, computational fluid dynamics (CFD) airflow simulations, and histologic analysis of olfactory epithelium. Results: Five uCLND subjects were included: 18-23 years, three male and two female, four left-sided cleft and one right-sided cleft. All subjects reported moderate to severe nasal obstruction. Smell identification and threshold tests showed varying degrees of hyposmia. Nasal resistance was higher on the cleft side versus noncleft side measured by rhinomanometry (median 3.85 Pa-s/mL, interquartile range [IQR] = 21.96, versus 0.90 Pa-s/mL, IQR = 5.17) and CFD (median 1.04 Pa-s/mL, IQR = 0.94 vs. 0.11 Pa-s/mL, IQR = 0.12). Unilateral olfaction varied widely and was dependent on unilateral percentage olfactory airflow. Biopsies revealed intact olfactory neuroepithelium. Conclusions: uCLND-associated olfactory dysfunction appears to be primarily conductive in etiology and highly susceptible to variations in nasal anatomy. Clinical Trial Registration number: NCT04150783.

Endoscopic Sinus Surgery in Older Patients With Significant Medical Comorbidities.

BACKGROUND: Endoscopic sinus surgery (ESS) offers excellent outcomes for patients with chronic rhinosinusitis (CRS) in the general population. It is unclear whether older patients with significant medical comorbidities experience similar benefits. OBJECTIVE: The purpose of this study is to evaluate whether increasing medical comorbidity is associated with worse sinonasal quality of life outcomes after ESS in older patients. METHODS: This is a retrospective study of CRS patients 55 years or older who underwent elective ESS at an academic institution from July 2017 to June 2019. 22-Item Sino-Nasal Outcomes Test (SNOT-22) scores were gathered at baseline as well as at 3 and 6 months following surgery. Data on demographics, medical comorbidities, preoperative Lund-Mackay (LM) scores, and postoperative complications were extracted from the medical record. The Charlson Comorbidity Index (CCI) was calculated for each patient. Multivariate linear regression was used to evaluate a potential association between CCI and change in SNOT-22 scores at 3 months postoperatively. RESULTS: A total of 205 patients met inclusion criteria with a mean (SD) CCI score of 2 (2.4) and a CCI score range of 0 to 11. The mean (SD) LM score was 8 (5.3). Rates of asthma and nasal polyposis were 28.3% and 36.6%, respectively. The mean (SD) improvement in SNOT-22 scores at 3 and 6 months compared to baseline was 17.9 (19.7) and 20.9 (18.1) points, respectively. After adjusting for covariates, there was no significant association between CCI and change in SNOT-22 scores. CONCLUSION: Greater medical comorbidity is not associated with worse SNOT-22 outcomes postoperatively, although future studies are needed to determine if comorbidities are associated with higher complication rates. A multidisciplinary approach to perioperative care is critical in maintaining the safety and efficacy of ESS in this patient population.

Endoscopic Sinus Surgery for Cystic Fibrosis: Variables Influencing Sinonasal and Pulmonary Outcomes.

BACKGROUND: Endoscopic sinus surgery is a well-established treatment for chronic rhinosinusitis in patients with cystic fibrosis, though its benefits seem to be limited to improving sinonasal symptoms rather than affecting lung function. OBJECTIVE: This study aims to identify clinical and demographic factors that may influence sinonasal and pulmonary outcomes after surgery. METHODS: This is a six-year retrospective analysis of adult cystic fibrosis patients who underwent endoscopic sinus surgery at a tertiary care center. 22-Item Sino-Nasal Outcomes Test scores and mean forced expiratory volume data at baseline and three to six months after surgery were analyzed using t-test and stepwise regression with the following covariates: age, gender, lung transplant, revision surgery, and pseudomonas on sinus culture. RESULTS: 119 surgeries were performed on 88 patients, with 69% on patients with transplant. The overall mean (Standard Deviation) improvement in 22-Item Sino-Nasal Outcomes Test score was 9.42 (18.15) for the entire cohort (P < .001). Pseudomonas on culture was associated with less improvement in sinonasal scores (P = .002). There was no significant change in forced expiratory volume after surgery (P = .94). Revision surgery (P = .004) and older age (P = .007) were associated with less favorable change of pulmonary function on stepwise regression (P = .002). There was no correlation between change in sinonasal scores and pulmonary function. CONCLUSION: Although surgery was associated with a clinically and statistically significant improvement in sinonasal scores in cystic fibrosis patients, patients with pseudomonas may experience less benefit. Revision surgery and older age may be associated with less favorable pulmonary outcomes. Awareness of such variables may help when deciding which cystic fibrosis patients should undergo surgery.

Aging-related olfactory loss is associated with olfactory stem cell transcriptional alterations in humans.

BACKGROUNDPresbyosmia, or aging-related olfactory loss, occurs in a majority of humans over age 65 years, yet remains poorly understood, with no specific treatment options. The olfactory epithelium (OE) is the peripheral organ for olfaction and is subject to acquired damage, suggesting a likely site of pathology in aging. Adult stem cells reconstitute the neuroepithelium in response to cell loss under normal conditions. In aged OE, patches of respiratory-like metaplasia have been observed histologically, consistent with a failure in normal neuroepithelial homeostasis.MethodsAccordingly, we have focused on identifying cellular and molecular changes in presbyosmic OE. The study combined psychophysical testing with olfactory mucosa biopsy analysis, single-cell RNA-Sequencing (scRNA-Seq), and culture studies.ResultsWe identified evidence for inflammation-associated changes in the OE stem cells of presbyosmic patients. The presbyosmic basal stem cells exhibited increased expression of genes involved in response to cytokines or stress or the regulation of proliferation and differentiation. Using a culture model, we found that cytokine exposure drove increased TP63, a transcription factor acting to prevent OE stem cell differentiation.ConclusionsOur data suggest aging-related inflammatory changes in OE stem cells may contribute to presbyosmia via the disruption of normal epithelial homeostasis. OE stem cells may represent a therapeutic target for restoration of olfaction.FundingNIH grants DC018371, NS121067, DC016224; Office of Physician-Scientist Development, Burroughs-Wellcome Fund Research Fellowship for Medical Students Award, Duke University School of Medicine.

Persistent post-COVID-19 smell loss is associated with immune cell infiltration and altered gene expression in olfactory epithelium.

SARS-CoV-2 causes profound changes in the sense of smell, including total smell loss. Although these alterations are often transient, many patients with COVID-19 exhibit olfactory dysfunction that lasts months to years. Although animal and human autopsy studies have suggested mechanisms driving acute anosmia, it remains unclear how SARS-CoV-2 causes persistent smell loss in a subset of patients. To address this question, we analyzed olfactory epithelial samples collected from 24 biopsies, including from nine patients with objectively quantified long-term smell loss after COVID-19. This biopsy-based approach revealed a diffuse infiltrate of T cells expressing interferon-γ and a shift in myeloid cell population composition, including enrichment of CD207+ dendritic cells and depletion of anti-inflammatory M2 macrophages. Despite the absence of detectable SARS-CoV-2 RNA or protein, gene expression in the barrier supporting cells of the olfactory epithelium, termed sustentacular cells, appeared to reflect a response to ongoing inflammatory signaling, which was accompanied by a reduction in the number of olfactory sensory neurons relative to olfactory epithelial sustentacular cells. These findings indicate that T cell-mediated inflammation persists in the olfactory epithelium long after SARS-CoV-2 has been eliminated from the tissue, suggesting a mechanism for long-term post-COVID-19 smell loss.

Persistent post-COVID-19 smell loss is associated with inflammatory infiltration and altered olfactory epithelial gene expression.

Most human subjects infected by SARS-CoV-2 report an acute alteration in their sense of smell, and more than 25% of COVID patients report lasting olfactory dysfunction. While animal studies and human autopsy tissues have suggested mechanisms underlying acute loss of smell, the pathophysiology that underlies persistent smell loss remains unclear. Here we combine objective measurements of smell loss in patients suffering from post-acute sequelae of SARS-CoV-2 infection (PASC) with single cell sequencing and histology of the olfactory epithelium (OE). This approach reveals that the OE of patients with persistent smell loss harbors a diffuse infiltrate of T cells expressing interferon-gamma; gene expression in sustentacular cells appears to reflect a response to inflammatory signaling, which is accompanied by a reduction in the number of olfactory sensory neurons relative to support cells. These data identify a persistent epithelial inflammatory process associated with PASC, and suggests mechanisms through which this T cell-mediated inflammation alters the sense of smell.

Rapid tissue prototyping with micro-organospheres.

In vitro tissue models hold great promise for modeling diseases and drug responses. Here, we used emulsion microfluidics to form micro-organospheres (MOSs), which are droplet-encapsulated miniature three-dimensional (3D) tissue models that can be established rapidly from patient tissues or cells. MOSs retain key biological features and responses to chemo-, targeted, and radiation therapies compared with organoids. The small size and large surface-to-volume ratio of MOSs enable various applications including quantitative assessment of nutrient dependence, pathogen-host interaction for anti-viral drug screening, and a rapid potency assay for chimeric antigen receptor (CAR)-T therapy. An automated MOS imaging pipeline combined with machine learning overcomes plating variation, distinguishes tumorspheres from stroma, differentiates cytostatic versus cytotoxic drug effects, and captures resistant clones and heterogeneity in drug response. This pipeline is capable of robust assessments of drug response at individual-tumorsphere resolution and provides a rapid and high-throughput therapeutic profiling platform for precision medicine.

International consensus statement on allergy and rhinology: Olfaction.

BACKGROUND: The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O). METHODS: Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus. RESULTS: The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies. CONCLUSION: This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Sinus Irrigation Penetration After Proposed Modified Draf IIa Technique in a Side-to-Side Cadaveric Model.

BACKGROUND: Endoscopic sinus surgery (ESS) is an effective intervention for patients with medically refractory chronic rhinosinusitis. Frontal sinusotomy is the most challenging part of ESS, with one of the key outcomes being access for topical irrigations. OBJECTIVE: The purpose of this study is to compare irrigation penetration into the frontal sinus following Draf IIa versus modified Draf IIa frontal sinusotomy. METHODS: Four fresh cadaver heads were used in this experiment. Draf IIa was performed on one side of each head and a modified Draf IIa on the contralateral side. This proposed modification consists of a Draf IIa combined with an agger nasi punch-out procedure and partial trimming of the vertical lamella of the middle turbinate back to the posterior table of the frontal sinus without drilling the beak. Each head was irrigated with methylene blue-dyed water and recorded by rigid endoscopy through an endonasal view (EV) of the frontal sinus and frontal trephination view (TV). Two blinded rhinologists scored the extent of staining (using an ordinal scale of 0 to 3) for each side. A case report where the modified Draf IIa was performed is also described. RESULTS: After modified Draf IIa sinuosotomy, the mean score for the EV was 2.125 and for the TV was 2, versus 0.875 and 0.625 for traditional Draf IIa, respectively. There was a statistically significant increase for both EV (p = 0.019) and TV (p = 0.018) after modified Draf IIa. CONCLUSION: In our cadaveric model, this procedural modification improved penetration of postoperative irrigations into the frontal sinus. This simple technique may be easily adapted into frontal ESS when indicated.

General Complications after Endoscopic Sinus Surgery in Smokers: A 2005-2016 NSQIP Analysis.

BACKGROUND: Exposure to cigarette smoke has been associated with a higher incidence of postoperative complications across a variety of surgical specialties. However, it is unclear if smoking increases this risk after endoscopic sinus surgery (ESS). Because complication rates after ESS are relatively low, a large national database allows for a statistically meaningful study of this topic. METHODS: The American College of Surgeons National Surgical Quality Improvement Program (ACS-NSQIP) dataset from 2005 to 2016 was analyzed. Patients who underwent ESS were identified. Thirty-day postoperative complication rates between smokers and nonsmokers were compared. Complications included infection, thromboembolic events, reintubation, readmission, acute renal failure, and cardiovascular events. RESULTS: 921 patients who underwent ESS were identified. 182 (20%) were smokers and 739 (80%) were nonsmokers. 609 patients underwent outpatient ESS, while 312 patients underwent inpatient ESS. A total of 12 patients experienced postoperative surgical site infections involving the deeper tissues beyond the wound (organ/space SSI). On univariate analysis, smoking was associated with a higher incidence of organ/space SSI (P = .0067) and pulmonary embolism (P = .0321) after ESS. On multivariate logistic regression, smoking was associated with increased odds (4.495, 1.11- 8.17, P = .0347) of organ/space SSI after ESS. CONCLUSIONS: This study demonstrates an association between exposure to cigarette smoke and potentially serious surgical site infections in the 30-day postoperative period after ESS. Our findings may help when counseling smokers who are considering ESS. Further study is required to understand the nature of these infections and ways to prevent them.Level of Evidence: 2c ("health outcomes").