I also think we didn't really appreciate how immunity actually works. There is this misconception that immunity means a disease can't touch you at all. But in reality, it just means your propability to fight it off quickly is much higher. In reverse, the propability to catch anything is much lower. If everybody's propability to catch something is low, the chance of it spreading is very low. For some diseases, where the incident cases are very low, and immunity in the population is very high, the propability of catching it is basically zero. Also, some viruses seem to be easier to defeat, once you have some immunity, while other still have a good chance of infecting you.
If you have small children, you know how often they get ill. This is because they have very little prior immunity to the hundreds of diseases we get bombarded with every day. Luckily, they also have an incredibly active immune system, so they are usually fit again very quickly. It is likely that they pick up the same viruses multiple times, but as we don't really test in the majority of cases, we don't know what they are. By the time they are grown up, they have build up enough immunity to the various diseases to only get ill now and then.
