Bob and Sally go to get their annual Flu vaccine at the public clinic. Every year, the two go together and neither have contracted Influenza since they began five years ago.
This time, while he was getting his shot, he says to his nurse, “These shots are great. I haven’t been infected with the Flu for years, despite at least some of my co-workers getting sick every year.”
His nurse finishes his injection and then says, “Well, you might have gotten infected, but you’ve didn’t get sick.”
“What do you mean? Isn’t that the same thing?”
“Actually,” says the nurse, ” it’s not.”
Explain what the nurse means by ‘infection’ and ‘getting sick’ being different things. Include, in your explanation, why it is that a vaccine might not prevent organisms from getting into your body and even into your cells, but that they can still fail to make you ill. What cells and molecules are involved in protecting you in this way?