HSV infection causes several distinct medical disorders. Common infection of the skin or mucosa may affect the face and mouth (orofacial herpes), genitalia (genital herpes), or hands (herpetic whitlow). More serious disorders occur when the virus infects and damages the eye (herpes keratitis), or invades the central nervous system, damaging the brain (herpes encephalitis). Patients with immature or suppressed immune systems, such as newborns, transplant recipients, or AIDS patients are prone to severe complications from HSV infections. HSV infection has also been associated with cognitive deficits of bipolar disorder, and Alzheimer's disease, although this is often dependent on the genetics of the infected person.
In all cases HSV is never removed from the body by the immune system. Following a primary infection, the virus enters the nerves at the site of primary infection, migrates to the cell body of the neuron, and becomes latent in the ganglion. As a result of primary infection, the body produces antibodies to the particular type of HSV involved, preventing a subsequent infection of that type at a different site. In HSV-1 infected individuals, seroconversion after an oral infection will prevent additional HSV-1 infections such as whitlow, genital herpes, and keratitis. Prior HSV-1 seroconversion seems to reduce the symptoms of a later HSV-2 infection, although HSV-2 can still be contracted. Most indications are that an HSV-2 infection contracted prior to HSV-1 seroconversion will also immunize that person against HSV-1 infection.
Many people infected with HSV-2 display no physical symptoms—individuals with no symptoms are described as asymptomatic or as having subclinical herpes.
Bell's palsy
Although the exact cause of Bell's palsy, a type of facial paralysis, is unknown it may be related to reactivation of herpes simplex virus type 1. This theory has been contested, however since HSV is detected in large numbers of individuals who never experienced facial paralysis, and higher levels of antibodies for HSV are not found in HSV-infected individuals with Bell's palsy compared to those without. Regardless antivirals have been found to not improve outcomes.
Alzheimer's disease
HSV-1 has been proposed as a possible cause of Alzheimer's disease. In the presence of a certain gene variation (APOE-epsilon4 allele carriers), HSV-1 appears to be particularly damaging to the nervous system and increases one’s risk of developing Alzheimer’s disease. The virus interacts with the components and receptors of lipoproteins, which may lead to the development of Alzheimer's disease. Without the presence of the gene allele, HSV type 1 does not appear to cause any neurological damage and thus increase the risk of Alzheimer’s. Herpes simplex virus type 1 DNA is localized within the beta-amyloid plaques that characterize Alzheimer's disease. It suggests that this virus is a major cause of the plaques and hence probably a significant aetiological factor in Alzheimer's disease.