Bird Flu Alarm

Understanding Avian Influenza

Why the bird flu virus is less deadly but more dangerous

People are rather confused over news that the bird flu virus has now mutated to a less lethal, but far more dangerous, form. It seems like a contradiction: If it's less lethal, shouldn't it be less dangerous?
No. The mainstream press isn't explaining this very well, so let me cover the basics. It has to do with evolutionary biology, or what you might call microbiological Darwinism, which basically involves how these viruses survive, thrive and get passed on from one person to the next.

If you're a really deadly virus -- like Ebola, which kills 90 percent of the people infected -- then you're actually not very good at spreading from one person to the next. Why? You kill your host too quickly. You're so deadly that your host dies before you get a chance to be infectious.

In order to be a pandemic, a virus must be highly infectious; it must be able to spread from one person to another in an undetectable way. When a virus becomes less-immediately lethal, it is able to survive in the host in an undetectable state, for a longer period of time. This is what makes viruses really, really dangerous: A dangerous virus is not lethal to one individual; rather, it can exist in a hidden state and be passed from one person to the next. It's the contagiousness of a virus that makes it dangerous.

Let's say you're a virus and you consider "success" to be wiping people out. Obviously, viruses don't have that sort of thought process, this is just a way to explain their strategies. If you're a virus and you're trying to infect and kill people, you're going to be far more "successful" if you have a low kill rate but infect a billion people, rather than having a very high kill rate and only infecting 10 or 20 people. If you are a very deadly virus in the Congo, for example, and you manage to wipe out a small village, even though you were rather horrifying to the village and fatal to those people, you as a virus haven't been very successful. Why? You wiped out the village; there's nobody left to spread it. Now, again, of course viruses don't think this way: They don't have plans, they don't have strategies -- this is just evolutionary biology in play.

On the other hand, let's say you are a virus with a very small kill rate -- you only kill one or two percent of your hosts -- but you're highly infectious. You, as this type of virus, can easily spread from one person to the next. Since 98 or 99 percent of the people who are infected with you won't die from it, they can walk around cities, airports and football stadiums and spread you to all the other hosts out there. If you are that kind of virus, you're going to be a lot more "successful" in spreading.

In the history of infectious disease, the most deadly viruses, in terms of the total number of people killed worldwide, were highly infectious, not necessarily highly lethal. If you look at the 1918 so-called "Spanish" flu (which really wasn't from Spain, but that's another story), the virus did not have a kill rate anywhere near 90 percent, or even 70 percent. I believe it was well under 20 percent. But this virus was good at spreading from one host to another, which is what made it extremely dangerous.

Understanding Avian Influenza