When you’re dealing with an infectious outbreak, there’s only one question you really need to ask.
Let’s say you catch the disease.
How many people will you give it to before you die?
That’s what the white coats call the reproductive number. R-zero.
Most of the diseases you got when you were a kid, and the ones you didn’t get because someone with your best interests at heart stuck you in the ass or the arm with a needle: they’re high r-zero. Measles. Mumps. Chicken pox: if you get chicken pox, you’re going to make ten other people itchy and scratchy, on average. That’s why they’re kids’ diseases: you can barely avoid them, so you catch them early. They’re endemic in the population.
That doesn’t matter too much if all you have to show for your ordeal is a few poxy scars. But it’s more of a problem if it’s going to leave you gammy-legged or sterile. Or dead. In those cases, you want to force r-zero down. Isolation’s one way to do it: keep sufferers away from everyone else and let the disease burn itself out.
But what you really want is a vaccine. And you want everyone to take it. Not only does the jab protect the vaccinee, but it’s hard – hopefully impossible – for the disease to reach susceptible people to infect in a vaccinated population. So the r-zero falls. Actually, it’s then termed r-vacc, or something, but let’s not worry about terminology right now, huh?
But the higher the r-zero, the more people you need to stick to stop it. Don’t want your kids to have the MMR because you’re still scared of that autism bullshit? Your choice, I guess. But if you don’t want them to get measles (r-zero: 14) you’d better hope that nine out of ten of their playmates’ parents were more responsible than you.
Stuff that’s low r-zero is usually easier to handle. Smallpox (r-zero:3) was pretty easy to wipe out: inoculate two-thirds of the population and you’re there. Polio (r-zero:6) is tougher: you need to get a more than four out of five. That’s why it was still hanging around in what used to be Nigeria and India years after it had vanished everywhere else in the world.
It’s never that simple, of course. People used to be scared of Ebola and lassa fever and all those sorts of diseases. Ebola basically makes people explode. All that black blood, all of it supercharged with virus. One drop, and it’s all over for you. But it’s not airborne; you have to be in physical contact with a sufferer to catch it. So r-zero for Ebola is only about two. It’s actually pretty easy to control: all you have to do is stop trying to help people.
There’s no vaccine, so isolation’s the only way. Leave them to die as soon as they show symptoms, and they’re gone so damn fast the virus doesn’t get a chance to spread.
“Catch it, kill it, bin it”: remember that? It works just as well for the infected as it does for the virus itself.
Now flu, there’s a different problem. Flu’s r-zero is about the same as Ebola’s. The problem is, it spreads quick but shows up slow. You have plenty of time to spread the disease before anyone knows you’ve got it. So by the time we knew about Z, it was already everywhere.
We didn’t have a vaccine. So it was down to isolation and treatment with antivirals. Which seemed, thank God, to work. Z’s r-zero was supposedly just over one. Enough to persist, not enough to spread. And if you got victims into isolation quickly, that meant r-zero of less than one: unsustainable. The outbreak would fizzle out.
A lot of people got sick, a few died. But most got better.
Or so we thought.
It took us longer than it should have to catch on to the fact that people who’d caught Z and got better… hadn’t. They just looked as though they had. They were walking around, still talking to people, still spreading the disease. All the time their bodies were just going through the motions, their brains rotting away in their heads. And we didn’t know it was happening.
(Maybe they did, though. Maybe that’s why they’re so damned eager to get hold of other peoples’ brains.)
If you’ve never heard of Typhoid Mary, look her up.
So Z’s r-zero wasn’t one. It was enormous. Off the chart. That means that by the time we’d caught on, it was way too late to put it back in the bottle, even if we’d known how. You’d have had to inoculate an insanely high proportion of the population to stop it spreading. And we still didn’t have a vaccine.
It’s amazing how long you can keep on thinking: it couldn’t happen here. First, you accept someone’s gonna die on the other side of the world. Then that someone’s gonna die in your country. Then someone you know. Then someone in your family. And then someone in your house.
Those are the worst. The mothers, the fathers, the brothers, sisters, grandparents and friends. First, you put them in a bedroom, follow the directions, leave food and medication outside the door for them to pick up. Maybe you listen for their footsteps, for the rasping of their breath that shows they’re still alive, still able to feed themselves.
And then the breathing turns into gasping. Moaning. Something doesn’t sound quite right. You start to worry about what’ll happen if they try to leave. You find yourself locking the bedroom door from the outside. By now you’ve started to hear that bad things are happening in locked bedrooms all over the country, but you don’t dare look. You don’t have the will or the equipment to deal with what you might see.
And the moaning turns into screeching, babbling. And the scratching at the door turns into pounding. And you’re wondering how long it will take Daddy or Grandma or little Billy to starve to death. And then you’re hoping it’ll be soon. Not for their sake, but for yours.
A full-blown zedhead – they don’t know what they’re doing. When they attack, it’s not even like an animal. It’s just savagery. Those bedroom doors didn’t hold them back long.
And if they’re strong, you die. Quickly. Sucks to be you, but that’s the end of that.
But if they’re weak, you’re just wounded.
You get away. But you catch the disease.
And if you’re wounded, you’re weak, and so you just wound your own victims.
And they get away, but catch the disease.
So what happens, weirdly, is this: survival of the weakest. If you’re a weak zedhead, your victims survive to carry the virus, and their victims survive, and so on. And the disease spreads. If you’re strong, the people you meet end up as mincemeat, and all that ripping and tearing burns you out quickly. So the disease dies out.
The zedheads were getting weaker and weaker. And the weaker they got, the more the virus spread. Until they got so weak that they were just as likely to win a fight as lose one. At that point, the selection pressure eased off.
So over time, we came to an arrangement. The most successful strains of the virus were the ones that didn’t drive their hosts to destroy themselves and others. They left their hosts just about functional, and their hosts’ victims just about functional. The virus’ reproductive number was managing itself upwards.
Until eventually, we had a population where Z was endemic. Kids get it almost before they can walk. Successful strains of the virus left the nurturing instinct intact, too. Most of the time, they go about their business; sometimes, they have violent outbursts. Just enough to keep the virus spreading.
It should. If you overlook the minor detail that the population has been quite literally decimated, it’s pretty much exactly the way things were before the virus hit.
Except we’re all zombies now. ##