First off, let me just throw it out there that this is entirely a theory. This is not proven, just my opinion. But it absolutely makes sense to me. So, lets get started. 

Some of you may remember that in science class, you learned about the ideas of natural selection. If not, it is defined as such: 

Natural selection — the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations. It is a key mechanism of evolution.

In other words, natures mechanism for killing off the “weak” ones, and initiating evolution. But only the good evolution. The “bad evolution”, or the evolution that does not benefit the species in question will be voided due to natural selection. 

Now, with modern medicine natural selecction seems to be disabled. People who carry the genes of cancer, diabetes, etc… are being passed on. They live, when nature told them they shouldn’t. It all gets passed on. The “bad evolutions” or bad mutations don’t get removed from the chain. 

How does that sound, eh? Not that they aren’t meant to live, it’s just that they pass on the genes that would have normally been removed from the chain. They got it from someone, and so did they, and so did they, and so on. 

*Note: This is not meant to piss anyone off, it is just my theory.