The answer would be C. -PENIA
The answer is viruses change their form when the immune system knows how to kill it and thats how they trick the immune system and viruses are much smaller than bacterias
<span>Edward Jenner discovered a vaccination against [smallpox] by infecting a boy with [cowpox]. He noticed that milkmaids who got cowpox never got smallpox and developed his theory from there. He is the father of the study of immunity and immunisation.</span>
