Influenza virus changes every year. The flu shot is developed by doctors that go to Asia, try to figure out the strains of influenza they think will be the most virulent, then devise a shot that will cover those strains. Yes, there's more than one strain of flu, every year. Sometimes they're right on target, other years they're far enough off the mark that people that have gotten the shot still end up catching the flu strain that's going around.
Smallpox still exists in the world, but luckily, only in the laboratory. Military members still get inoculated for it, because it can be used as a bio-weapon. Supposedly, the USA has enough stockpiled doses of the shot that they could keep the civilian population from being devastated by it, but personally, I'm glad that I'm old enough to have been inoculated as a kid, and again when I joined the navy (years ago).
The only reason we don't see smallpox breaking out anymore is because finally enough people were inoculated for it, that it could no longer survive in the wild. However, if it were ever released as a weapon, it could spread through the population that hasn't been inoculated against it fairly rapidly, unless they got the shots available in a timely manner.