Smallpox is a virus that causes a rash that then turns into painful blisters. The last known case of smallpox was in 1977. What is the best explanation for the eradication of smallpox? People are exercising more than they used to.
Genetic testing determines who is more prone to smallpox.
There was an increase in antibacterial products.
An effective vaccine was developed.

An effective vaccine was developed

Explanation:

(I actually learnt about this is history!)

Smallpox was around 75-100 years before 1977, and people continued to die from it especially when there were severe outbreaks (thousands of people lost their lives). The main technique for vaccination was to make a small cut in the patients arm or wrist, and then to rub a smallpox scab into the open wound (inoculation). However this was not as effective as it sometimes, brought infections into the wound, but the success rate was still pretty high.

Edward Jenner was convinced that there could be a more effective way of vaccinating people against the smallpox. Jenner discovered (from a milk maid!), that those that have been infected with cowpox won't catch small pox, as they are from the same family. Instead of small pox being the vaccine, Jenner used weakened cowpox pus instead. This was much effective as the side effect of having cowpox mild, and the patient made a full recovery.

Jenner put this cowpox vaccine in injections so that an open wound is not needed.

By the 1850's the vaccine was made compulsory, but many were against it: still believing that miasma (foul air) was the cause of the small pox. It was made compulsory or face a fine in the mid 1950's, hence small pox was eradicated by 1977

Hope this helps!