We need to be repeatedly vaccinated for influenza viruses because

a. the viruses attack only the helper T cells, thereby suppressing the immune system.

b. the viruses alter their surface proteins and thus avoid immune recognition.

c. influenza doesn't actually generate an immune response; the "flu" is actually an inflammatory
response
d. influenza viruses are too small to serve as good antigens. There are two ways to avoid getting chicken pox when you are older: by catching it when you're young or by receiving a vaccination. How do each of these methods work, and in what ways, if any, are they similar?