Stool samples were collected as part of previous clinical studies and were anonymized. This study was approved by the University of California San Francisco Committee on Human Research. Samples from Nigeria and Tunisia were collected as part of the World Health Organization's poliovirus eradication program from children with non-polio AFP between the ages of 4 months and 15 years. Stool samples from healthy contacts of case patients from Tunisia were matched for age. Stool samples from Nepal were obtained from adult travelers and resident expatriates with diarrhea with no known pathogens detected by standard microbiologic testing for enteric bacteria; enzyme immunoassay for rotavirus, adenovirus, astrovirus, Giardia and Cryptosporidium; and reverse transcription PCR for norovirus. Stool samples from healthy, asymptomatic control persons were collected from the same population. Stool samples from the Minnesota Department of Health are from individuals with diarrhea and healthy individuals, matched for age and residential area code.