Lysis plaques of lambda phage on E. coli bacteria.

By itself a viral particle is just an amalgamation of macromolecules: genetic material in the form of either DNA or RNA, a protein capsid, and in some cases a lipid envelope. A virus has no metabolism. It is an inert particle. However, when a virus infects a host organism it can make use of the host cell's components and metabolism to produce multiple copies of itself, which assemble in the cell. Although the life cycle of viruses can differ greatly among species, there is a general common narrative involving attachment, penetration, uncoating, replication, assembly and release.

There are many examples of human diseases caused by viruses including the common cold, influenza, chickenpox, Ebola and AIDS. For future doctors, the basic microbiology of viruses is an obvious focus. From life cycle to classification, the details of viruses must be well reviewed before the MCAT. Additionally, certain viruses are valuable tools as vectors in recombinant DNA experiments, so there is an additional aspect to study which is the important role viruses play in the molecular biology laboratory.