Not all vaccination programs are completely successful in eradicating a disease. One that was, is the smallpox vaccine.

This disease was caused by the variola virus. 12-30% of sufferers died while many who recovered were often blinded. In 1967, WHO (the World Health Organisation) vaccinated more than 80% of the worlds population who were at risk and when a case was reported all possible contacts in the area were vaccinated (ring vaccination).

Eradication in rural areas proved a challenge, but the last case occurred in Somalia in 1977 and in 1980, WHO declared the world free of smallpox.

Reasons for the success of the vaccine included:

1. The variola virus did not mutate and change its antigens.
2. It was made from a live harmless strain of a similar virus, so it mimicked a natural infection, multiplying and continually presenting the immune system with a large dose of antigens.
3. It could be freeze-dried and kept for six months aiding distribution.
4. Infected people were easy to identify.
5. It was easy to administer and the disease did not linger in the body.
6. Smallpox does not infect animals.

Less successful vaccination programs have included those against measles, tuberculosis, malaria and cholera.

This disease offers the promise of eradication if worldwide surveillance was followed-up by vaccination.

However, so far it has failed because:

1. A poor response to the vaccine has been shown by some children, who need boosters.

2. High birth rates and shifting populations make following-up cases difficult.

3. Migrants and refugees may spread the disease.

4. Measles is highly infective and 95% immunity of a population is required to prevent transmission.

5. The vaccine only has a 95% success rate.

This disease was once thought to have been eradicated, but is actually showing a resurgence.

The reasons include:

1. Some TB bacteria are resistant to drugs used to treat them because they can mutate.

2. AIDS can allow TB to infect an individual due to their compromised immune system.

3. Poor housing and homelessness lowers peoples' natural resistance.

4. There have been breakdowns in the TB control program.

5. It is actually caused by two different bacteria with two different antigens, which can live inside human cells, making them hard to fight.

6. It can be carried in cattle.

This is a disease caused by Plasmodium, a protoctist (eukaryotic) that has hundreds or even thousands of different antigens. It also has three different stages in its life cycle, meaning that developing a vaccine is incredibly challenging and has not yet been achieved.

This disease is caused by Vibrio cholerae, which can live in the intestines where antibodies - produced by vaccines that are injected - cannot get to it. An oral vaccine is in development.