When you get sick with the flu you get infected with flu viruses and they make lots of new flu viruses, but those new viruses are not exact copies of the old ones. They have mutations in them. A lot of those mutations are harmful.
Carl ZimmerThe reason that viruses are so hard to fight, the reason for example we need a flu virus every year is that they evolve very fast.
Carl ZimmerThe only kinds of ways we have to deal with viruses are old school, so vaccines for example are very effective, but the first vaccines were invented in the 1700's, so we're talking about technology that is over 200 years-old.
Carl ZimmerMedicine allows people to live who would otherwise die, so antibiotics will let people survive infections that they might be otherwise very vulnerable to and even little things might make a big difference, so I wear eyeglasses because my eyes aren't particularly strong, before there were eyeglasses someone at my age would probably not be good for much.
Carl ZimmerI wouldn't be a very good hunter without these glasses. I'm not a very good hunter with these glasses, but I'd be even worse without them, so that would put a crimp in how many kids I could have, so all of these medical advances have at least in some parts of the world blunted natural selection.
Carl ZimmerParasites are not only incredibly diverse; they are also incredibly successful. There are parasitic stretches of DNA in your own genes, some of which are called retrotransposons. Many of the parasitic stretches were originally viruses that entered our DNA. Most of them don't do us any harm. They just copy and insert themselves in other parts of our DNA, basically replicating themselves. Sometimes they hop into other species and replicate themselves in a new host. According to one estimate, roughly one-third to one-half of all human DNA is basically parasitic.
Carl Zimmer