Viruses cause all kinds of illnesses, some potentially fatal or with severe side effects, others less virulent-seasonal flus, measles, polio, herpes, hepatitis, shingles, HIV/AIDS, Ebola. Their infectiousness makes them a particular concern to communities, public health programs, and policy makers. Being infected by a deadly virus affects social relations between people, sometimes leading to extreme stigma or silence, other times giving rise to health activism and global health campaigns. Viruses are hard to treat, many antiviral medications are expensive, and these medications often suppress but do not cure infections. Given this, health programs tend to emphasize prevention, an approach that requires people to engage with health programs when they are not sick. Vaccination is a preferred option; inoculation against smallpox beginning in the early nineteenth century, for example, led to the worldwide eradication of this disease. This public health success continues to inspire global immunization programs for children and adults, but shifting attitudes and variable adherence to childhood immunization have made it less easy to approach the goal of eradication for other infections. And for some deadly diseases, including HIV and Ebola, we do not have yet effective vaccines.