The focus of this thesis is to describe the epidemiology of two causes of infectious diseases that are relevant to the Middle East and North Africa (MENA): arboviruses and gastrointestinal pathogens. Manuscript 1 is entitled “The Epidemiology of Dengue, Chikungunya, and Yellow Fever in the Middle East and North Africa: Systematic Review and Meta-Analysis.” In this work, we reviewed published records across the MENA region to describe the human prevalence, human incidence, and vector infection rates for dengue (DENV), chikungunya (CHIKV), and yellow fever (YFV) viruses. Through a systematic search, we identified 145 human prevalence measures for the study viruses in the MENA region: DENV (n=104), CHIKV (n=24), and YFV (n=17). Seroprevalence measures >20% were identified for DENV (range 0-60%) and CHIKV (range 0-42%) in the Red Sea region and Pakistan, while evidence for YFV infections was confined to Sudan. Meta-analyses across all studies in the MENA region estimated general population mean IgG seroprevalence for DENV at 8.6% (95% CI: 5.3-12.6%), CHIKV 4.3% (95% CI: 1.1-9.1%), and YFV 3.7% (95% CI: 0.2-10.4%). However, there was considerable heterogeneity in effect size as well as high variability in subregional study coverage, target study populations, and diagnostic methods among reported studies. This study provides a detailed characterization of the epidemiology of these arboviruses in the MENA while identifying current research gaps and priorities for future research. Manuscript 2 is entitled “Multiplex PCR for Detection of Gastrointestinal Pathogens in Migrant Workers in Qatar.” In this work, we conducted a prospective observational study to understand the clinical characteristics and infectious causes of diarrhea among migrant workers in Doha, Qatar. Seventy-five male workers coming to the Qatar Red Crescent Worker’s Health Center outpatient clinic or emergency department were enrolled in the study. Surveys were administered to all subjects and the prevalence of 23 different stool pathogens was determined by multiplex PCR (FilmArray® Gastrointestinal PCR). Salmonella was the most prevalent pathogen and was detected in 27% of all subjects, followed by enteropathogenic E. coli (21%), enteroaggregative E. coli (15%) and enterotoxigenic E. coli (12%). In a multivariable analysis, a triage heart rate ? 90 beats per minute was the only significant predictor of a positive PCR result (OR 4.5, 95% CI 1.1-18.7). Use of multiplex PCR enabled the detection of gastrointestinal pathogens in 57% of cases overall, illustrating the utility of this diagnostic tool in epidemiologic studies of infectious diarrhea.