The human immunodeficiency virus.

HIV is a complex retrovirus. Like some other viruses it infects host cells for life, but unlike other viruses it appears to do so every time. Its elaborate genetic regulation enables it to remain relatively dormant, replicating steadily but slowly. On appropriate stimulation, it is capable of explosive up-regulation, releasing high numbers of new infectious virus. It replicates in an error-prone way, constantly changing its structure to improve its infectivity while presenting the host's immune system with a constantly moving target.