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Chapter 18

PLASMODIUMAdvertisement

Imagine dying from a mosquito bite.

About two million people every year do just that, thanks to a parasite called Plasmodium. Here's how it works:

When an infected mosquito bites you, Plasmodium is injected into your bloodstream. It moves through your body until it reaches your liver, where it stays for about a week. During that time it changes into a new form - sort of like a caterpillar turning into a butterfly.

Did I say butterfly? Actually, it's more like a microscopic tank. Plasmodium grows treads that allow it to crawl along your blood vessel walls, and it develops a sort of missile launcher on its head. This launcher helps the parasite blast its way into one of your red blood cells.

Inside the blood cell, Plasmodium is safe, hidden from your immune system. But it stays busy. It consumes the insides of the cell and uses them to build sixteen copies of itself. Those burst forth and go on to invade more of your blood cells, where they each make sixteen more copies of themselves...

You can see where this might become a problem. This problem is called malaria.

Getting malaria sucks. As your blood cells are consumed by Plasmodium, you get chills, then a high fever that comes back every few days. Your liver and spleen expand, and your urine turns black with dead blood cells.

It gets worse. All those blood cells are supposed to be carrying oxygen through your body. As they get turned into plasmodium-breeding factories, the oxygen stops flowing. Your skin turns yellow, and you become delirious. If your malaria remains untreated, you'll eventually go into a coma and die.

But why is Plasmodium so nasty? Why would a parasite want to kill you, when that means that it too will die? This seems to go against the law of optimum virulence.

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Here's the thing: Humans can't give one another malaria, because most people don't bite each other. So to infect other humans, Plasmodium needs to get back into a mosquito.

This is trickier than it sounds, because when a mosquito bites you, it only sucks a tiny, mosquito-size drop of blood. But plasmodium doesn't know which drop of blood will get sucked, so it has to be everywhere in your bloodstream, even if that winds up killing you.

In this case, optimum virulence means total domination.

But plasmodium isn't completely lacking in subtlety. Sometimes it takes a break from killing you.

Why? Because if too many humans in one place get malaria at the same time, it might wind up killing them all. This would be very bad for Plasmodium; it needs a human population to keep breeding. So every once in a while, Plasmodium plays it cool. In fact, one strain can hang around inside you for as long as thirty years before it makes its move.

It lets you think that you're okay, but it's still there, hiding in your liver, waiting for the right moment to unleash its engines of destruction.

Clever, huh? pqdm.com