Inside your body, a nonstop war rages. On one front, your immune system regulates the beneficial bacteria in your gut, mouth or skin. On another front, it's fighting off germs, including bacteria or viruses with bad intentions.
Fortunately, your body stands ready for this infectious invasion. Each infection is a race between your immune system — mostly involving white blood cells — and the invader.
If your body has seen the virus or bacterium before, it already has a game plan and a head start on the race, and the immune system typically wins out.
Sometimes, though, disease-causing germs appear that are so new and foreign that the immune system can't fight back fast enough and has to start from scratch. The COVID-19 virus, a new form of the familiar coronavirus, is one example. Your immune system is fast, but it takes a week or two to catch up, and you feel sick during that time. Sometimes your body cannot fight back fast enough and you need medical help.
For new diseases, call in the scientists. They work to identify the new threat and develop therapies — including vaccines — that can bolster your immune system's ability to fight back.
To do this, vaccines imitate certain characteristics of the organism causing the infection — without actually causing the infection. This teaches the immune system to identify the infection, form protective antibodies and recruit important immune cells to fight it off before you get seriously sick.