Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses.

Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate using the replication proteins of a host cell.

envelope — lipid bilayer that envelopes some viruses. capsid — protein coating of the viral core. asymptomatic disease — disease where there are no symptoms and the individual is unaware of being infected unless lab tests are performed. lysogenic cycle — type of virus replication in which the viral genome is incorporated into the genome of the host cell.