Everybody dies eventually. Even if you manage to avoid bear attacks or grievous bodily harm, something in your own body will go wrong sooner or later. Usually later though, since age is the single biggest risk factor for disease. Strangely, we don't really understand why our bodies deteriorate as we age, and so we don't know how to prevent it, but there are a few scientists who think they know how to stop the relentless march of time, and they figured it out by accident.
The researchers in question were studying why some cells in our bodies become cancerous. They believed turning off a gene called BubR1 could be the culprit, so they genetically engineered mice with BubR1 deactivated. But the mice didn't develop tumors as expected. Instead they aged rapidly, and by the time they were just three months old they had thinning hair and cataracts. The mice lost weight, developed heart problems, and died much earlier than normal. When the scientists examined their bodies, they noticed that the mice had accumulated a particular type of cell called "senescent cells."
Senescent cells were once healthy normal cells, but they've been damaged or their DNA has mutated. To avoid passing that damage onto their daughter cells, they stop replicating and start putting out signals for the immune system to come and clean them up. They're not exactly dead, but they're not doing anything either. If the immune system cleans up these cells, it's no big deal, but it appears that over time, the immune system stops responding and the senescent cells build up.