Why do you have to get a TB shot?

Immunization to the disease

The vaccine is said to work in 40% of people given it

It saves money as the cost to treat a person with it is higher than the cost of giving a person a vaccination to help prevent them from catching the disease.

It would be worth giving the vaccine to homeless people and other groups that are at risk of getting the disease, as it is pretty cheap and otherwise could kill the person so it may save lives.