During winter break you came down with a horrible cold. Then, during spring break, you came down with another horrible cold! In fact, you have probably had the common cold dozens of times in your life. If having a disease is one way to gain immunity, why have you not become immune to the common cold?
a. There are at least 200 different viruses that can cause the common cold, and they change over time.
b. Actually, you have developed immunity to the common cold and get sick only from bacterial infections.
c. Only non-specific immunity helps you fight the common cold, and specific immunity is not an effective defense against viruses.
d. The pathogen that causes the common cold attacks and kills memory cells, continually "erasing" the immune system's memory of it.
e. The rhinovirus that causes the common cold has no surface antigens and so can move through the body without being detected.
