A viral infection is caused by viruses-tiny infectious agents that invade cells in the body to replicate and spread. Symptoms and severity can vary widely depending on the virus and the affected body system. Influenza, rhinovirus, COVID-19, RSV are respiratory viruses; Norovirus, rotavirus, adenovirus are gastrointestinal viruses; Herpes simplex virus, chicken pox, shingles and measles are skin-related viruses; Hepatitis B and C, HIV are bloodborne viruses; Rabies, West Nile virus and polio are neurological viruses; Dengue, Zika and Ebola are viruses that attack the whole system.