Viruses are smaller than bacteria and need living hosts, like humans to multiply. Otherwise, they cannot survive. When a virus enters your body, it invades some of the cells and takes over the cellular machinery, and redirects it to produce the virus. Viruses cause common infectious diseases such as the common cold, flu and warts. They also cause serious diseases such as HIV / AIDS, smallpox and Ebola.

For most viral infections, treatments can only help with the symptoms while your immune system fights the virus. Antibiotics do not work for viral infections. There are antiviral medications to treat some of them. Vaccines can help prevent you from getting many viral diseases.