A virus is a tiny infectious agent that can only replicate inside the living cells of an organism. Viruses can infect various life forms, including animals, plants, and bacteria. They are responsible for many diseases, such as the common cold, influenza, and COVID-19, caused by the SARS-CoV-2 virus. When a virus enters a host, it hijacks the host's cellular machinery to produce more virus particles. This process can lead to cell damage or death, resulting in symptoms of illness. Vaccines and antiviral medications are common methods to prevent or treat viral infections, helping to control their spread in populations.