Surprised that "rabies vaccination" was not discussed in this context or in comments yet. Rabies infection when it sets in after an unfortunate contact with affected animal (bats, dogs, wild animals) is almost 100% fatal. But if rabies vaccination (post-exposure prophylaxis) is provided within a short period of time it is almost 100% effective. Unfortunately, 99% of mortality due to rabies is in developing countries due to it being endemic in dogs in these places. I agree with author that nothing in science is 100% that is why we follow P<0.05 rule. There is always a 5% chance of being wrong in any clinical experiment. But in certain situations such as vaccination to prevent rabies infection you can be 99.99% sure that it works.