A virus is a microscopic infectious agent that replicates only inside living cells of other organisms. Viruses cause many diseases including colds, influenza, COVID-19, measles, and HIV/AIDS. Unlike bacteria, viruses cannot be treated with antibiotics—antiviral medications exist for some viral infections, but treatment options are more limited than for bacterial infections. Prevention through vaccines is crucial for many viral diseases. Public health responses to viral outbreaks include surveillance, vaccination campaigns, public education, and sometimes quarantine measures. The COVID-19 pandemic demonstrated how viral diseases can affect every aspect of society. Understanding that antibiotics don't treat viral infections helps reduce inappropriate antibiotic use that contributes to antibiotic resistance.