Here's a fact of life that may sound odd at first: Cell death is absolutely necessary for any living thing to develop, grow, and survive. Hmm, you say, so my human cells have to follow a regular schedule of dying off for me to live on? Exactly right.
Our bodies are made up of quadzillion different kinds of cells — bone cells, muscle cells, liver cells, brain cells, skin cells, blood cells, and way too many others to mention. Cells die naturally by a process called "apoptosis," which in Greek means "falling off." This well-timed cell death keeps everything in working order and helps fight off attacks from invaders, like bacteria and viruses.
If you think of apoptosis as cell suicide, you can consider "necrosis" more in the line of cell murder. In necrosis, an outside influence injures a cell. The cell that's been treated poorly swells, the membrane around it splits, and nasty chemicals leak out all over a bunch of nice neighboring cells. Those cells are now damaged, too.
When cells follow the right program of apoptosis, everything runs smoothly. When there's not enough apoptosis going on, problems occur. Cells that ignore their death sentences can create cancers, viral infections, and auto-immune diseases. Likewise, cells that die before their time — because of too much apoptosis — are the bad guys in cases of AIDS, Alzheimer's, liver disease, and heart attacks.
Studies of all this stuff will tell us how to make sure cells don't kick off too soon or too late, which will contribute to controlling diseases. Meanwhile, in the time you've taken to read this response, millions of cells in your body have followed orders to self destruct. (And you didn't feel a thing!)