A virus is a collection of nucleic acid, either DNA or RNA, surrounded by a protein coat, which replicates within the cell of another organism. Viruses are interesting in that they have no cell of their own and rely on the host cell’s machinery to replicate their nucleic acid. Viruses also use the host cell’s machinery to create capsids, the protein shells that surround and protect the viral nucleic acid.
The life cycle of a virus consists of attaching the virus capsid to a cell membrane of the host cell and then inserting the viral DNA or RNA into the cell. After the host cell has copied the viral nucleic acid and manufactured the virus, the host cell is destroyed. This action releases thousands of capsids and allows new viruses to spread to other cells. The majority of vaccines available in the United States are targeted against harmful viruses.