By contrast, the smallest bacteria are about 400 nanometres in size. The protein capsid provides protection for the nucleic acid and may contain enzymes that enable the virus to enter its appropriate host cell. Viruses are classified on the basis of their nucleic acid content, their size, the shape of the capsid, and the presence of a lipoprotein envelope. The virus's developmental cycle begins with the entrance of the particle's nucleic acid, and in some cases its proteins, into a susceptible host cell. Bacterial viruses adsorb and firmly attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host.
Animal viruses enter host cells by a process called endocytosis.
This pattern is called a lytic type of infection. Bacteriophages sometimes, however, show a different pattern of infection, called the lysogenic, or temperate, type. In a lysogenic infection, the viral genome is integrated into the chromosome of the host cell and becomes known as a prophage, which replicates in concert with that chromosome prior to cell division. In such cases, no progeny virions are produced and the host cell remains intact. The viral genome, however, is being passed on to each new generation of cells that stem from the original host.