A virus is made up of genetic code surrounded by a protein coat. They are the most common biological entities on Earth, outnumbering bacteria 10 to 1. Because a virus is technically not a living organism (at least not in the traditional sense), it can only thrive and reproduce by infecting living cells within humans, animals, and plants. This process often kills the infected cells, which can cause damage to the host and create disease.
Examples of diseases caused by viruses include COVID-19, the common cold, influenza, and measles, among many others. Unlike bacteria, which are living organisms, viruses cannot be killed by antibiotics. Only antiviral medicines and vaccines can eliminate or prevent viral diseases.