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Epidemiology (how we know mosquitoes don't spread AIDS)

Epidemiology is one of the most important branches of investigative medicine. Whenever any new disease appears, epidemiologists are usually the first to study it. But while many people have heard the word, very few people actually understand what it is and how it works.

The reason that epidemiology is powerful is that it can yield practical answers about a disease without having any theory behind them, because it's based completely on collection and evaluation of evidence. The answers come from the evidence directly.

Suppose you're walking in the woods and you find a footprint in the ground. It's more or less round, about 4 inches across, and has toes with claws on the end.

You can't necessarily identify what made it, but you can certainly eliminate a lot of things. It wasn't made by a deer or cow or horse or anything with hooves because they don't have toes or claws. It wasn't made by a raccoon or mouse because it's too big. It wasn't made by any bird I've ever heard of because their footprints are completely different. See, already we've excluded a lot just from observation. Probably it was made by some sort of large mammal; possibly a big cat (puma) or a bear. Were you a naturalist, you might be able to identify it exactly, but even a layman can narrow the field down quite a lot. And once we've concluded that it wasn't made by a horse, nothing we discover later will change our minds about that; there's no way a horse would make a footprint with toes and claws. Though we may not be able to identify what it was, or may later learn more about it, we can conclusively exclude a lot of things.

That, in essence, is what epidemiology is doing. In a nutshell, an epidemiologist gathers as much data as possible about victims of the disease and also about people in the same area who are not victims, then sits down and compares them all to each other. If something is common to all victims but not found among the normals, it's a good candidate for being related to the disease. If it's not common in the victims, or if it's common among both victims and non-victims, it's not likely to be related.

Epidemiology actually predates the germ theory of disease, and its first major success at eradicating a disease was in London. Cholera was one of the great killers of the 18th and 19th century; it spread across Europe and finally reached London. There were many outbreaks of the disease, and each one killed thousands of people. No-one knew what caused it, no-one knew what to do about it, no-one knew how to avoid getting it because no-one knew how it was transmitted. A lot of crazy things were tried which didn't work.

A statistician working for one of the major insurance companies had a great deal of data about people who'd died of the disease and decided to analyze it to see what he could discover about them. He tried checking things like the professions or trades of the victims, what their income was, and stuff like that and found nothing. Then he started plotting their homes on a map, and struck pay dirt. He found a "cluster" of cases in one particular part of London, where surrounding areas had few cases.

Now in London in the 1830's, when this was happening, was a lot different city than the one we know now. In particular, it didn't have what we think of as modern plumbing. Running water in your house was a luxury granted only to the very rich. Most people got their water from communal water pumps placed at intervals around the city, and lugged the stuff home to drink and use for other purposes. This particular cluster of cases completely surrounded one of those pumps, and the number of cases tapered off the further away from the pump you got.

The statistician got suspicious, and convinced the city authorities to remove the handle from that pump so that it couldn't be used anymore. The locals complained because now they had to walk a lot further, to other pumps, to get their water. But the cases of cholera in that neighborhood tapered down to nearly none.

It was found that a local cesspool (no sewage system, remember) had been leaking and had contaminated the water in that pump with sewage. Why that mattered, no-one knew, but the other pumps around there had not been contaminated.

Ultimately, the City of London spent a lot of money to build a real system to carry sewage away, and put serious effort into cleaning up the water supply. And once they did, cholera epidemics went away and never came back, much to everyone's relief.

Which only left one question: why had it worked? No-one at the time knew. Clearly sewage in drinking water had something to do with it, but that was all they knew. Yet they were able to stop the epidemics nonetheless.

It wasn't until much later that the explanation was found. Cholera causes its victims to suffer from extreme diarrhea, and the flow contains live bacteria which are the disease pathogen which causes cholera. If you ingest such bacteria, you have a good chance of getting the disease yourself. The cesspool which had leaked had been used by people who had the disease, and the contaminated water contained cholera bacteria. People drinking that water were thus exposed to the disease and had a very high chance of also getting it. When forced to walk to other pumps, the water they brought back was not contaminated, and they were no longer being exposed to the disease to the same extent, so there were far fewer cases, and eventually none.

But that all came a couple of decades later. The point is that our hero had identified the means by which the disease spread and determined how to prevent it -- all without having the slightest idea what the disease pathogen actually was.

And that, at its roots, is how epidemiology works.

On an informal level, that kind of thing had been done before, but never quite that systematically. Syphilis had long since been identified as being passed through sexual intercourse, for instance, although no-one knew what caused it.

Advance forward 150 years, and certain people are beginning to suffer from a strange condition where their immune systems decline to the point where they ceased to work entirely. No-one knew why. At first it was very rare, but the number of cases kept growing. In the US, it was mostly homosexual men who were suffering from this, though it was also springing up among hemophiliacs.

The epidemiologists got on the case and started studying the victims, and those around them. Just as we were able to eliminate lots of animals as sources of our footprint, they were able to eliminate a lot of potential ways that the disease was spread. Casual contact? Conclusively eliminated by study of families of hemophiliacs, where family members didn't seem to get the disease. Coughing and others breathing that air? Contact with saliva? All eliminated. Ultimately they concluded that it required direct transmission of blood or certain other body fluids directly into the blood of someone else to give them the disease. The hemophiliacs were getting it because of injections of clotting factor, which was derived from human blood. The homosexual men were getting it because they were receiving anal sex, which tends to cause the anus to bleed, and thus opened a pathway for exposure to the semen of other men.

At this point, no-one had any idea whether it was caused by a bacterium, a virus, a multicelled parasite, or something else entirely. No-one had the slightest idea what the pathogen was. But they were able to determine how it moved from one person to another simply by analyzing all the information they collected. They didn't find all the ways; only the ways by which the victims they studied had gotten it.

Only much later was the actual pathogen found, the Human Immunodeficiency Virus (HIV), and they're still working out all the details about just how it does what it does; it's a complex little beastie.

But in the mean time, due solely to epidemiology, they were able to begin control measures to start slowing the spread of the disease, even though they didn't know what caused it. This consisted mainly of education of the gay community and encouragement of use of condoms, and of efforts to clean up the blood supply by eliminating donors who were "at risk".

Much later, when the HIV virus had been identified, it became possible to develop a test to determine if a given person had been exposed to the disease, and once this was in place, the blood banks began using it routinely to check every pint of blood they received.

Now part of the initial study was not just that they identified how those particular victims had gotten the disease, but that they identified a whole lot of ways that they hadn't. That's perhaps more important.

A quick lesson in logic, to explain something called the "contrapositive". Consider the following two statements:

If A, then B

If not B, then not A

Those statements have the same truth value at all times. If either of those statements can be demonstrated to be true, then the other must also be true. If either is demonstrated to be false, then the other must be. Example: If I stand naked in the ocean, I will get wet. I'm not wet, therefore I'm not standing naked in the ocean.

Keep this in mind, because we're going to be using it. Here's the general idea: If the disease can be passed a certain way, then certain people will have the disease. If they don't have the disease, then that particular way doesn't pass the disease. We don't need to know what causes the disease to make that conclusion.

AIDS didn't just occur in America; it began to pop up in other places too. In particular, it started getting very common in parts of Africa (where they called it "thin disease" because of the wasting effect it had on its victims). So the epidemiologists went to Africa and started collecting data on the victims there who were getting it, and on the people around them who weren't.

And again, while they didn't immediately identify the actual means of transmission, they were able to immediately exclude a lot of them. One obvous one was the possibility that the disease was spread by blood-sucking insects such as mosquitos or biting flies.

Mosquitos bite everyone. If mosquitos could spread the disease, then everyone in a given village would have about an equal chance of getting the disease. (Which is the case with malaria, for instance.) But that's not what they found for AIDS. When they studied their data, they found that some groups were getting the disease far more often, and some groups hardly ever got the disease, even though they all lived in the same area. Which brings us to our logic:

If biting insects were spreading the disease, nearly every segment of the population would be getting it in about the same proportions.

Not every segment was getting it in the same proportions. Therefore it wasn't being spread by biting insects.

Again, it's important to emphasize that this is conclusive based on the information they collected about people who did and who did not get the disease. More important, it doesn't require any theory about exactly what causes the disease (and at the time they didn't know). It's a direct deduction from observed evidence; it doesn't require a theoretical explanation. It's a fact.

Just as we can be certain that our animal track with claws wasn't a from a horse, we can be certain that HIV isn't spread by biting insects. It isn't necessary to explain why this is so, because the result is conclusive based on direct evidence. Nothing more is needed.

In the African studies, the epidemiologists ultimately demonstrated that the primary means of transmission of the disease was heterosexual sex. Often that was anal (which is common there as a means of birth control) but often it was vaginal.

Back in the US, the disease started showing up among intravenous drug abusers, but the epidemiologists were really hitting their stride by that point because they had learned so much already, and in fairly short order were able to demonstrate that it was being transmitted by people sharing needles to do their drug injections. As a result, in some areas which were more openminded, needle-exchange programs cut down on the spread rate among drug abusers.

And all of this was done before the HIV virus was actually identified, or any direct study of it had been done. That's the power and value of epidemiology.

Some epidemiologists are gutsy as all hell. When a new disease pops up, they're right in there visiting victims, taking blood samples, talking to them and to others in the area -- and if it should turn out to be a disease which is passed by casual contact, they have a chance of getting it. Some of the epidemiologists who studied Ebola appear to have been astoundingly lucky, because ebola is passed through casual contact and it has over a 90% fatality rate.

That said, it's important to note that since their conclusions are direct deductions from evidence, later theories will not change them. Epidemiologists are collectors of facts, and in science when theories and facts are in conflict, facts nearly always win.

Epidemiology is not flawless; it can only find what is there. When they were studying the homosexual victims of AIDS, they didn't identify heterosexual sex as a means of transmission because none of the victims they studied ever did that. But when epidemiology excludes something, it stays excluded. That's the important point.

The study of hemophiliac cases conclusively proved that casual contact with a person having HIV does not significantly increase your chance of getting the disease. That fact hasn't changed and won't change unless the disease mutates. The study in Africa conclusively proved that HIV is not transmitted by biting insects. That fact, too, hasn't changed and won't change unless the disease mutates.

Such a mutation seems to be extremely unlikely; viruses just aren't that versatile. HIV has now been genetically sequenced and they know exactly what information it carries. It does in actuality mutate nearly constantly in a victim, but only in certain areas having to do with the protein coat of the virus, which permits it to evade the immune system to some extent. It can't change that too much, though, or it loses the ability to infect cells, and a virus cannot reproduce without being inside a cell and taking over the cell's mechanisms.

Its characteristics are now really quite well known (though there's still much more to learn), and the precautions which must be taken to prevent spread are also well understood. HIV is actually not very infectious, and if you take the proper precautions you can lower your chance of getting infected to nearly zero. They're fairly simple and have been widely publicized, so I won't repeat them here.

And the things which were determined to not be risk factors still aren't. You can't get HIV from a mosquito bite, or perhaps it might better to say that the chance of that happening is immeasurably low.

It is possible for you to be killed by being struck on the head by a meteorite. In fact, historically it's actually happened. But it's a vanishingly small chance, and I don't know anyone who wears a steel helmet to protect themselves against such a thing. Equally, it may be possible for HIV to be transmitted by a mosquito. But the chance is so low that it's not worth taking precautions against it.

The epidemiological evidence for this is absolutely conclusive. More study and analysis of the HIV virus, and new theories about it, won't suddenly make mosquitoes start transmitting the disease. It's not going to change.

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