A latent infection is a situation in which a virus is present in the body, but it remains dormant, not causing any overt symptoms. The patient is still infected with the virus, and he or she can pass the virus on to others when they are exposed to the dormant virus. Latent infections can also be activated, causing symptoms and illness to emerge again. A classic example of a latent infection is herpes simplex, which periodically flares up to cause cold sores before going dormant again.
People sometimes confuse latent infections with latency. Latency or clinical latency is one of the things which occurs during the incubation period of an infection, in which the causative agent is present in the body and multiplying, but not causing symptoms. The virus involved in clinical latency is not dormant, as is the case with these infections, but fully active and causing problems for the host organism. Eventually, the virus will move out of latency and start causing detectable symptoms, alerting the host to the fact that an infection is occurring.
Some infections can never be fully flushed from the body, becoming latent with the use of medications and other measures to control the virus and inhibit replication. In these cases, the infection may periodically flare up in response to environmental cues. Latent infections can also be caused when a virus mutates, becoming impossible to eradicate, or when a course of treatment is not completed, allowing a virus to remain dormant in the body.
A number of viruses are characterized by causing latent infection, allowing the virus to ebb and flow in the body in cycles as the environment changes. From the point of view of the virus, the ability to go dormant is critical, as it allows the virus to retain a host while becoming dormant when conditions are hostile or unpleasant for the virus. These infections can also be very hard to detect, or to manage.
In addition to causing problems for the host by periodically flaring up and causing an array of symptoms, these infections can become more sinister. Several viruses have been linked with out of control cell division, which is presumably caused by a scrambling of the viral and cell DNA which leads to crossed wires and rampant division of the cell. Latent infections can also become a serious problem when a patient becomes immunocompromised, as the infection may manifest when the patient's immune system passes a critical point.