In The Andromeda Strain, they rode in on a satellite.  In Outbreak, they came from the jungles of Africa.  In the news, we are continually hearing about the bird flu coming from southeast Asian chicken markets.  I think it's time for some inner reflection, literally  Chances are, you are infected with at least one of the following critters: Herpes Simplex Virus, Epstein Barr Virus, Cytomegalovirus, Neisseria meningitidis (leading cause of bacterial meningitis), Adenovirus.  And here is the real kicker (this is drum roll worthy): every single one of us carries around hundreds of copies of ancient retroviruses in our DNA.

And I bet you feel fine, huh.  Yup.  They have adapted. They have learned how to keep you unaware of their presence.  Think about what viruses and bacteria need - to make as many copies of themselves as possible  They can't do this ad infinitum in you because that would eventually kill you off.  They need you to sneeze, touch doorknobs, kiss and have sex, thus allowing them entry into other people where they can continue to multiply.  Generally speaking, people isolate themselves when they are sick, making transmission that much harder for these microbes.  Therefore, some of the most successful human pathogens - those that have been with us the longest - have adapted to a lifestyle that keeps you smiling and mingling.

What about my immune system, you might ask, why isn't it showing these party crashers the door?  In some cases it literally can't.  Take Herpes Simplex Virus for instance.  You might only notice it on your girlfriend's lip, but that's not where it lives.  It lives in a nerve cell roughly located by her sideburns.  Only occasionally does it pop out, travel down to the tip of the nerve cell (near her lips), and replicate in the cells of her mouth.  Unfortunately for you there isn't a damn thing her immune system can do about it in the nerve cell.  Nerve cells are kind of a no-go for your immune system; there are powerful limits to its ability to attack any of your nervous system (otherwise your immune system could destroy your brain). In the cells of her mouth is a different story - her immune system attacks and mops up.  But it's still in the nerve, laughing.  Not only that, in the nerve cell it doesn't express any genes for your immune system to see.  Once you got it, you aren't getting rid of it.  This sort of thing is also a problem with HIV.  Every time doctors have taken patients off triple drug therapy, no matter how long they have been on it and even though the virus is at undetectable levels, it comes back.  Somehow a tiny amount of the virus is able to hide somewhere and avoid detection, and unfortunately you only need just one virus particle to get it all going again.

Now for the soup du jour: a virus that is so successful that we do all of the work for it.  Retroviruses have invaded our DNA.  How do we know this?  One of the hallmarks of retroviruses is that they replicate by using 3 characteristic genes: Gag, Pol and Env.  Some (like HIV) have more than these three, but all retroviruses have at least these three.  When the human genome project was finished they found that at least 8% of our DNA is derived from retroviruses  Considering that there is about 6.5 feet of DNA inside each one of your cells, that means that a little over 6 inches is retroviral in origin.  Multiply 6 inches by however many billion cells you have in your body and I would say that is one successful virus (actually there are several different kinds, but you get the point).

Fear not, for none of these is actually infectious so far as we can tell.  Some of them are so mutated that they couldn't possibly form a virus.  But a lot of them do express their genes and that brings up the freakshow: at least one retroviral protein is actually functional and plays a role in the development of the placenta.  You remember, that thing that kept you alive when you were in the womb.  Retroviral genes had a hand in making that  So each night I go to the crib, look at my son sleeping like an angel and say "Thanks retroviruses, good job!"