I'm a big disease geek. In another life, this might have made me a brilliant doctor. In this life, where I experienced the usual amount of girl-discouragement about my interest in sciences and I'm terrified of needles, it pretty much makes me boring at parties. Although I cannot get a dictionary to substantiate this, I remember reading that the definition of an epidemic was that for every person who got better, more than one person was getting sick. If that seems like a weird thing to grab my imagination, I'm not sure any of the rest of this entry will make sense to you either. For the sake of wading through the rest of it, imagine that you're terribly afraid of dinosaurs. You cope with your fear by renting Jurassic Park over and over. In the movie, you might get scared, but you can stop and start it any time you want, rewind, skip parts, make the dinosaurs smaller. And you know how it ends.
Being a queer teenager in the mid-to-late eighties meant that AIDS was flickering on the edge of my consciousness when they were still calling it GRID--Gay Related Immune Deficiency. It meant just ducking a wave of grief, and it meant coming to believe that there is no end, at least not in my lifetime. Enter the Black Death, at a safe distance of about six hundred years. I've read a lot about both.
I was pretty surprised, therefore, to see a PBS documentary last night that told me something I'd never heard about either.
Yersinia pestis, the bacteria that causes bubonic plague, most commonly gets into the human body via a flea bite. (There's a whole gross thing here about starving fleas' digestive tracts and bacterial replication, but you probably don't want to know.) Once inside, all it has to do is wait for your immune system to respond. White blood cells get sent to the site of infection, but instead of getting squished, Y. pestis invites itself into the cells and hitches a ride into your lymph system to start wreaking merry havoc. That's what you're seeing with the buboes that give the disease its name: swellings at the armpits, throat and groin where the lymph nodes are overwhelmed. Diabolical, i'nt it? So how does it manage?
Why, using your chemokine receptors. At this point it starts to get pretty technical, so I hope that any microbiologists reading my blog will forgive me for minor inaccuracies while I try to make this into, y'know, English. When your body is injured, it sends chemokines as a protein S.O.S. A gene called CCR5 picks up this call, and Y. pestis just sticks its foot in the door. The frustrating thing is that CCR5 seems to be redundant; there are other genes that do the same thing without the added bonus of an excruciating and often fatal infection.
But wait! It's a bird! It's a plane! It's a mutation!
Yep. In some people CCR5 exhibits a mutation known as delta 32. And guess what? Y. pestis can't use the mutant gene. Yay, mutants! The interesting part is that the Black Death and subsequent outbreaks of bubonic plague in Europe may have helped to select for this particular mutation. Having one copy of delta 32 slows down the rate of bacterial replication, giving your immune system more time to come up with some tricks of its own. At least in theory, this improves your chance of survival. Two copies of delta 32 makes you the winner of the world's least fun lottery, leaving you unscathed by plague and free to help dig mass graves. While somewhere between a third and a half of the population of Europe died, the survivors passed on their sometimes-wonky genetic legacy.
Although HIV is a virus rather than a bacterium, it uses a similar bag of tricks, binding itself to your CD4 cells and using CCR5 as a co-receptor. Uh, doorway. So having delta 32 may actually confer some immunity to HIV, although I wouldn't go throwing out your condoms any time soon. There are other co-receptors available, and viruses are good at exploiting opportunities. Still, I sat in amazement for a while after the show was over.
How come I have all this free time? Well, I'm sick with that eternal mutant, the common cold. A cousin of the virus I contracted in August, it got the better of me, and don't think I'm not bitter. But I'm also kind of awed, as I drink my appalling amount of water, watch documentary television, and blow my nose, to think about my body ticking away. Synthesizing the right antibody. Kicking viral ass. At least, till the next time.