Lai, JianyuAcute respiratory infections (ARIs), which usually appear in the form of common colds and influenza, as well as outbreak brought on by emerging viruses such as SARS-CoV-2, result in millions of deaths and hospitalizations each year. Aerosols being exhaled by infected population and inhaled by susceptible population has been identified as an important transmission route for ARIs; yet few studies have compared the viral load in exhaled breath aerosol (EBA) in naturally and experimentally infected cases, as well as among various infections. The specific aims of this dissertation were: 1) examine the comparability of EBA viral shedding between experimentally and a full range of natural ambulatory influenza cases; 2) compare seasonal coronavirus, influenza, SARS-CoV-2 Omicron, and other SARS-CoV-2 variants in terms of viral loads in exhaled breath aerosols; and 3) examine the relative efficacy of face masks, KN95, and N95 respirators as source control for SARS-CoV-2. We analyzed data from four studies that collected 30-minute fine (≤5 μm) and coarse (>5 μm) EBA samples using a Gesundheit-II sampler. Viral RNA load in EBA was quantified using real-time RT-PCR. Nasal inoculation of influenza virus A/Wisconsin/67/2005 showed lower EBA viral shedding compared to the natural influenza A H3 infections. Among the viruses studied, SARS-CoV-2 Omicron variants demonstrated the highest viral RNA loads in both EBA size fractions, emphasizing its superior spread capability via inhalation. Furthermore, while all masks and respirators showed significant reductions in viral RNA load in exhaled aerosols, the duckbill N95 respirators stood out, providing reductions of up to 99% and outperforming both surgical and cloth masks, and KN95 respirators. Given the evident transmission risk via inhalation for the studied viruses, measures such as masking and indoor air hygiene are crucial. The pronounced efficacy of N95 respirators highlights their importance in healthcare settings and places with vulnerable populations, especially during periods of heightened respiratory viral infections.enDissertationEpidemiologyacute respiratory infectionscoronavirusexhaled breath aerosolinfluenzarespiratorsSARS-CoV-2