Some of us have heard warnings from scientists and medical professionals about “antimicrobial resistance,” which is when microbes adapt to resist, or even ignore, medications we rely on for health care. Many people know antimicrobial resistance is a real threat, but not how it happens.

How do microbes like bacteria, fungi, parasites and viruses grow to become resistant?

Bacteria, a type of microbe, are single-celled life forms and reproduce by dividing themselves. In theory, each time a bacterium splits, that should result in two exact copies of the original, but that doesn’t always happen.

With every split, there’s a tiny chance that a copy will be a little different than its parent. A lot of the time, the change has no effect, or makes life harder for the copy. But sometimes, the change makes the copy more likely to survive, and pass the adaptation along to its descendants. This is how new strains and variants of microorganisms form.

If we use antibiotics to treat a bacterial infection and some of the bacteria survive thanks to an adaptation, they’ll multiply and pass that resistance down. The next time we try the same treatment, it will kill even fewer of the bacteria than before, since surviving bacteria passed on their adaptation. The same thing can happen with viruses, fungi and parasites, and the drugs we use to treat them.

Drug resistance can happen any time we treat bacterial or other microbial infections, but it’s more likely if the treatment isn’t carried out properly. That’s why you should always take antibiotics as prescribed.

Preventing drug resistance is just as important for animals as it is for people too. Use of antibiotics for pets and other animals can also lead to resistant bacteria.

Risks posed by drug-resistant microorganisms may be great, but there are ways we can help stop them before they develop.