What Makes a Disease Contagious?

People don’t normally catch colds from an animal, even your beloved dog or cat. Be careful when cleaning up after a sick friend, since viruses can live on countertops for a while.

A disease is infectious if it’s caused by a bacteria, virus, or other bug that gets inside you. It’s only contagious if people can pass it to each other. Not all infectious illnesses are contagious. For instance, malaria is spread by mosquitoes. But it can't jump from person to person. So it’s infectious, but not contagious.

When you are vaccinated, the shot mimics the disease and your body creates a natural defense to it. This gives you immunity. If you are exposed to the disease later in life, your body remembers and protects you.

Polio is a good example. The last time there was a natural outbreak that started in the U.S. was in 1979. But some countries still have it. So if a traveler brought it home and people stopped getting polio shots, it could spread.

You can find it in almost everywhere, including soil and dust. But you get the infection from things, not people. Hepatitis B and colds pass from person to person.

It depends how easily the illness spreads. Some illnesses can easily pass from person to person if they use the same things, like remote controls or phones. But you won't catch other diseases, like HIV, unless you come into contact with their blood or other bodily fluids.

No one has caught smallpox since 1977. Once one of the world’s most feared illnesses, it officially ended in 1979 thanks to a vaccine used worldwide. Samples of the smallpox virus still exist in only two labs in the world, just in case.

Although some, like Ebola, are life-threatening, others are mild. Take pinkeye, for example. It's a hassle and it will spread through a daycare center quickly. But it's very rare for it to become a major problem. "Contagious" is all about how a disease spreads, not how bad it is.