Mrs. Turner,
Think of it this way. There are viruses that want to infect us. That is what they do. That is how they reproduce. When a virus infects you, it's a battle between it and your immune system. The virus wins if it can replicate faster than you can kill it off. We fend off viruses all the time, but sometimes the virus wins.
If you watched the HBO series John Adams, you might remember when a sick cow was brought to their house, and Abigal Adams deliberately infected each of her children. There was a deadly smallpox epidemic in Massachusetts. By stimulating the immune system with a weaker version of the disease from the cow, her family was protected from the deadly version.
A couple of decades later,
Edward Jenner demonstrated that people who were infected with cowpox (a much less dangerous infection) gained immunity from smallpox. This was the humble beginning of humans gaining the upper hand against many viruses which had plagued, maimed, and killed us in large numbers throughout our history.
Since then we have developed numerous vaccines which train the immune system to fight nasty viruses. But some are tricky. Influenza, which kills tens of thousand of people in the United States every year, can evolve so much from one year to the next that we must try to predict which strains will be circulating next time, and make a new vaccine every year. The HIV virus is so tricky we haven't figured out yet how to train the immune system to fight it.
When a virus infects me and my immune system tries to fight it off, I prefer my defenses to be well prepared. Personally, in the battle between viruses and humans, I'm rooting for the humans. But then, as a human, I'm clearly biased.
