Are viruses alive? The reason that they're currently hanging out somewhere outside of the five kingdoms is that they're not made of cells, which people have long considered to be the basic unit of life. But they do reproduce and evolve, and they have the same kind of genetic material as living things. This has led many scientists to wonder whether we've got the definition of life right in the first place, and newer ways of organising the tree of life have been proposed to include viruses.

A virus is a pretty simple thing: it's just a shell, called a capsid, containing a piece of DNA or RNA. The cells of all living things use these same genetic materials as blueprints to make copies of themselves. What viruses don't have, though, is the machinery to make more viruses on their own. What use is a blueprint without a factory? To reproduce they have to inject their genes into another organism's cells and take them over, forcing them to pump out copies of the virus. Making the viruses often interferes with whatever the cell was supposed to be doing in the first place, so many viral infections cause some kind of illness. Diseases from the common cold and chicken pox to AIDS and ebola are caused by viruses.

Because you can't treat them with antibiotics, these diseases can be trickier to treat than ones caused by bacteria. The good news is that our bodies' immune systems do a pretty good job of fighting them off naturally, manufacturing antibodies to destroy them. If you've had a particular strain of a virus once, your body can probably deal with it easily if it comes around again, using the antibodies it learned how to make the first time— that's why you don't usually get chicken pox twice. If you catch a virus that spreads very quickly, though, or attacks your immune system itself, making enough antibodies in time can be a problem, and you can get really ill. Because of this, doctors vaccinate people against some viruses. This means that they inject a harmless version of the virus into a person, so that their body begins to manufacture antibodies. When the real thing comes along, bang! The antibodies are already in place and can destroy the virus before it makes us sick. There are also antiviral drugs which attack the viruses directly, but viruses evolve quickly and usually become resistant to them in time.

Virus	Description	Mask
Norovirus	Also known as the "winter vomiting disease", new strains of these pesky viruses cause a lot of people a lot of misery each year. If you'd like to learn more, have a look at the mask!