Viruses are the cause of much human misery. When you get sick, a virus is often to blame. Viruses cause:
And, of course, AIDS.
Viruses are very different from cells. For one thing, they are much, much smaller than most cells. HIV is about a million times smaller than the cells it infects. If HIV were the size of a walnut, a cell would be as big as a refrigerator.
Viruses have a much simpler structure than cells. Some are no more than a little DNA inside a hollow shell made of protein. Others, like HIV, have some extra parts they carry around inside. Later, you’ll read how HIV needs these pieces to reproduce.
In fact, viruses are so simple they cannot reproduce by themselves—they have to infect (get inside) a cell to do that.