OVERALL ABSTRACT Human astroviruses (HAstV) are a leading cause of infectious diarrhea. Yet, the current ?gold-standard? diagnostic test fails to detect 2/3 of the HAstV genotypes associated with human infection making it likely that HAstV infection is not only under-diagnosed, but more prevalent than appreciated. There has also been an increased appreciation that HAstV disease can range from asymptomatic to significant and even fatal systemic diseases beyond diarrhea including encephalitis and meningitis, particularly in high-risk populations suggesting that astrovirus-associated diarrheal disease is also not as straight-forward as dogma would have us believe. Yet, the role for distinct HAstV genotypes in disease is currently unknown and no studies to date have defined the host or viral factors connected with astroviral-associated diarrheal disease. Our proposed studies begin to fill this gap in knowledge by being the first work aimed at understanding HAstV-associated diarrhea: from epidemiology to basic virology. Strong preliminary studies suggest that diverse viral genotypes co-circulate in pediatric oncology patients with boys being four times more likely to be infected than girls, and that viruses isolated from patients have distinct in vitro phenotypes. Thus, the overall goals of our planned studies are to begin to understand HAstV-associated diarrhea in a novel high-risk population. Our central hypothesis is that the ability of astroviruses to cause diarrheal illness is largely mediated by their broad heterogeneity and distinct biological behaviors. To test our central hypothesis, we have proposed the following three interconnected but independent specific aims: 1. Understand the association between astrovirus infection and diarrheal disease. 2. Determine if diarrheal disease correlates with in vitro phenotypes. 3. Are there additional HAstV genotypes? We are uniquely suited to undertake this work given our expertise in astrovirus biology as well as having novel cohorts of pediatric oncology patients that are among the highest risk for having astrovirus infections with access to extensive metadata including onset/presence of diarrheal disease. Importantly, we have already demonstrated that our cohorts are infected with genotypes from all 3 clades, and now we will be able to determine their association with diarrheal disease. Intriguingly, there appears to be a sex bias with males more likely to be HAstV-positive. Extensive new genomic information will be generated that will be invaluable to many investigators. In the long term, these studies may reveal critical new information on the public health impact of viral enteric infections.