What are viruses?

Viruses are behind many types of illness, from mild colds to severe diseases like acquired immunodeficiency syndrome (AIDS) caused by the human immunodeficiency virus (HIV). Even COVID-19 itself is mild for some and severe for others. Viruses have been found all over the Earth. Researchers estimate that they are ten times more common than bacteria. These tiny germs, that are not even a cell themselves, invade other organisms and are an unavoidable part of our lives.