Why is it that some diseases, like the cold or flu, can happen every year, while other diseases, such as the chicken pox, only usually occur once in a person’s lifetime

Diseases like the flu or cold evolve each year where as diseases like chicken pox to quite evolve.

Explanation:

Since the flu and the cold evolve every year, they develop immunity to previous shots.  For example, if you get a flu shot, you get another one the following year because the antibody for the flu shot the first year does not work for the flu the next year.  This is simply because the flu virus constantly tries to find ways to develop immunity and thus they constantly mutate.

Diseases like chicken pox occur once in a person's lifetime simply because they are a different class of virus compared to the flu.  Their virus mutates very slowly and thus they can't develop immunity as fast.