True or False: Vaccines are designed to protect against invaders that are encountered rarely, not all the time
Not true. Vaccines were initially made for the highly contagious diseases that used to cause epidemics (which obviously means they were not rare). 

The efficacy of a vaccine depends on multiple factors which includes adaptability of the pathogen. 

The all-the-time encountered pathogen that the article is talking about is HIV; the reason for why no effective vaccine exists for it is because it has high mutation rate.