Technically, viruses are not members of any domain of life. They are considered here because, like bacteria, they are microscopic and can cause human diseases. Viruses are acellular particles that lack the properties of living things but have the ability to replicate inside living cells. They have no energy metabolism, they do not grow, they produce no waste products, they do not respond to stimuli, and they do not reproduce independently. In the view of biologists, they are not alive.

Viruses consist of a central core of either DNA or RNA surrounded by a coating of protein. The core of the virus that contains the genes is the genome, while the protein coating is the capsid. Viruses have characteristic shapes. Certain viruses have the shape of an icosahedron, a 20-sided figure made up of equilateral triangles. Other viruses have the shape of a helix, a coil-like structure. The viruses that cause herpes simplex, infectious mononucleosis, and chickenpox are icosahedral (solid with 20 faces). The viruses that cause rabies, measles, and influenza are helical (spiral-shaped).

Viruses reproduce only within living cells. They attach to the plasma membrane of the host cell and release their nucleic acid into the cytoplasm of the cell. The capsid may remain outside the cell, or it may be digested by the host cell within the cytoplasm. In the host cytoplasm, the DNA or RNA of the viral genome encodes the proteins that act as enzymes for the synthesis of new viruses. The enzymes use amino acids in the cell for protein synthesis and nucleotides from the host DNA for nucleic acid synthesis. The viruses obtain cellular ATP and use cellular ribosomes for additional viral synthesis. After some minutes or hours, the new viral capsids and genomes combine to form new viruses.

Once formed, the viruses may escape the host cell when the host cell disintegrates. Alternately, the new viruses may force their way through the plasma membrane of the cell and assume a portion of the plasma membrane as a viral envelope. In either process, the cell is often destroyed and hundreds of new viruses are produced.

Viruses can cause a number of human diseases, including measles, mumps, chickenpox, AIDS, influenza, hepatitis, polio, and encephalitis. Protection from these diseases can be rendered by using vaccines composed of weak or inactive viruses. A viral vaccine induces the immune system to produce antibodies, which provide long-term protection against a viral disease.