Molluscum contagiosum is the most common human disease caused by a poxvirus (a "family" of viruses including smallpox) infection and the number of cases appears to have increased significantly over the past few decades. In fact, one American study found an 11-fold increase in the numbers of patients visiting healthcare providers regarding molluscum contagiosum over 20 years. In the Unites States, the virus, MCV, accounts for 1% of all skin conditions diagnosed. It exclusively affects human skin and conjunctiva (the mucous membrane on the inside of the eyelid and the whites of the eye). Although the virus and the distinctive papules (raised areas of skin) it causes are found world-wide, the location of the lesions, the age, and the sex of those exhibiting symptoms vary by geographic location. It is more common in men than in women. There are four different types: I, II, III, and IV, but III and IV are rare, with one study finding that nearly 97% of cases were caused by Type I and over 3% by Type II. Type II is more common in HIV-positive individuals, where it comprises 60% of cases. Most outbreaks clear up completely on their own after between two to four months.
The molluscum contagiosum virus (MCV) is, as its name indicates, contagious. The presence of papules can be provoked by crab lice bites, tattooing, or accidental scratching. They usually occur when MCV is inoculated (placed where it can grow and reproduce) in a tiny abrasion in the skin. Once the virus has found an abrasion or injury to the skin, it can infect and enlarge skin cells and replicate (makes copies of) itself.
It appears that close physical contact with either the papules or materials that have been in contact with them is necessary to transmit it. Lesions (skin abnormalities) often spread from a baby's face to its mother's breast or face and neck or from sexual partner to sexual partner. Gym equipment, contact sports such as wrestling, swimming pools, and shared towels may also spread MCV, though experts are unsure if the virus is spread through the water or through contact with others and objects related to swimming and bathing. In male organ transplant patients, those living with HIV/AIDS, or individuals undergoing cancer treatment, the papules are often spread by injuries caused by shaving. Electrolysis can also spread papules to other parts of the body. Scratching or otherwise damaging the lesions can make it easier for the virus to autoinoculate (spread itself) as it frees MVC and allows it to infect other damaged parts of the skin if it is moved from one spot to another.
Since MCV can only live in the skin, once the bumps are gone, so is the virus. It is unlike herpes viruses, which can remain dormant for some time. If someone experiences a second outbreak, they have come into contact with the virus elsewhere. It does not appear that having the virus once builds immunity.
At this point in time, there is no vaccine for MCV.