Viruses are based on nucleic acids and proteins. They are at the frontier between life and organic chemicals. They are based on DNA or RNA. They have a protection for nucleic acids based on proteins.
As they have no cytoplasm, they have to live at expenses from other organisms or cells. They sometimes lose their protein during the infection and the nucleic acid enters the cell. It goes directly to the nucleus, where it can take control of it for it to make more viruses. It sometimes can change the cell's genome to incorporate into it's DNA. The cell then transmits the virus genes to its offspring.
After infecting the cell, the viruses can leave the cell by means of exocitosis (they are excreted to the exterior) or lisis (they activate the enzymes for cell's self destruction).
They cause a lot of diseases, some of them lethal. HIV is a virus that causes AIDS, there is a virus which causes flu. There is one called coxsackievirus, which causes infectious myocarditis, infectious pericarditis, and pleurodynia.