Which of the following diseases is not preventable by a vaccine?

Polio, Measles, and Tetanus are all preventable by vaccines. Polio is caused by a virus and can be prevented with the polio vaccine. Measles is also caused by a virus and can be prevented with the measles vaccine. Tetanus is caused by a bacterial toxin and can be prevented with the tetanus vaccine.

However, Malaria is caused by a parasite that is transmitted through the bite of infected mosquitoes. There is currently no vaccine to prevent malaria, although other preventative measures such as anti-malaria medication and mosquito nets can be used to reduce the risk of infection.

Therefore, Malaria is not preventable by a vaccine.