Definition
A virus is a tiny infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea. They consist of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Viruses are not considered living organisms because they cannot reproduce independently. Think of them as tiny pirates that hijack cells to make copies of themselves! They cause a wide range of diseases, from the common cold to more serious illnesses.