Is Small Pox Back?

Smallpox is a 12,000 year old disease which scarred mankind for centuries till an effective immunization campaign helped eradicate it. The disease killed millions and disfigured many. Dreaded as it used to be, the story of smallpox is also the story of man's victory over disease. It has been completely wiped off since 1970 and no case has been reported since. A very recent news (march 19, 2007) of a toddler in Chicago who developed serious infection after his father was immunized brings back memories of the disease and its devastating consequences. It is important to understand the basics of vaccination before we understand the significance of the isolated incident.

What is vaccination?
Our body has a brain where memory functionally resides. The immune system of the body also has a memory. Whenever a foreign agent(e.g. bacteria or virus) enters through skin or mouth or lungs, the body is faced with the task of containing the organism. A complex reaction ensues where the immune cells get activated from a dormant state to a very aggressive killer state all to protect the body. While a group of cells are busy killing the offender another group(the helper cells) have been busy taking down information about the organism make up. Reason: next time the organism strikes the body is ready with an army of cells which are custom made for the offender(bad news for the offender, now they are designed to kill it). This memory property of the immune system is utilized in vaccination/immunization. And this how once somebody contracts measles once does not get it again. But a vaccination does not involve actually giving the disease to protect the person from future attacks as this obviously is not desirable. So a weakened virus(similar to the dangerous actual wild virus) or a part (antigen) of a virus or bacteria is introduced into the body to make the immune system know the actual organism better and prepare for it in case there is an attack. The weak virus can sometimes create problems itself and this is what possibly happened in the recent incident.

How is Small pox vaccinated?
Small pox is of the type called Variola (major and minor). The vaccine is prepared from Vaccinia(cow pox) , a related but weak virus. It causes a mild disease but the body gets a taste of what small pox would be like. We owe it to Edward Jenner who discovered the vaccine and saved millions. But the whole exercise is a trade off. Vaccinia can in some cases( one in a million) cause brain fever(encephalitis), heart infection (myocarditis) and skin rash. People with eczema, pregnancy, HIV/AIDS should avoid the vaccine or the person who has been vaccinated recently(as such person in principal is infected with vaccinia and can transmit it to susceptible individuals). Such people are prone to have serious infections requiring isolation and monitoring.

The toddler in news had eczema and this is probably what caused him(made him susceptible) to have a serious rare condition called Eczema Vaccinatum.
The chances of small pox coming back is close to zero given the large number of people vaccinated.