['Pathogens are thought to regulate host populations. In agricultural crops, virus infection reduces yield. However, in wild plants little is known about the spatial and temporal patterns of virus prevalence. Thus, pathogen effects on plant population dynamics are unclear. Prevalence data provide necessary background for (1) evaluating the effects of virus infection on plant population size and dynamics and (2) improving risk assessment of virus-resistant transgenic crops.', 'We used ELISA and RT-PCR to survey wild Cucurbita pepo populations over 4 years for five viruses, aphid-transmitted viruses of the genus Potyvirus as a group and PCR to survey for virus-resistance transgenes. In addition, we surveyed the literature for reports of virus prevalence in wild populations.', 'In 21 C. pepo populations, virus prevalence (0-74%) varied greatly among populations, years, and virus species. In samples analyzed by both ELISA and RT-PCR, RT-PCR detected 6-44% more viruses than did ELISA. Eighty percent of these infections did not cause any visually apparent symptoms. In our samples, the virus-resistance transgene was not present. In 30 published studies, 92 of 146 tested species were infected with virus, and infection rates ranged from 0.01-100%. Most published studies used ELISA, suggesting virus prevalence is higher than reported.'] Are virus infections in wild plant populations both frequent and often unapparent?