“There is nothing permanent except change.”

—Heraclitus

Mutant swarm of influenza viruses

The final wall of defense, after barrier methods and the best attempts of the innate immune system have failed, is the “adaptive” (or “acquired”) arm of the immune system: the ability to make antibodies. Antibodies are like laser-guided missiles specific for a particular foreign invader. They are made by immune cells called “B cells” that arise in our bone marrow. We make a billion of these cells every day of our lives, each recognizing a specific target. Once one of these B cells is activated, it can pump out thousands of antibodies a second.²⁰² And some of them remember.

So-called “Memory B cells” retain a memory of past invaders and lie in wait for a repeat attack. If that same invader ever tried anything again, the body would be armed and ready to fight it off before it could gain a foothold. This is why once you get chicken pox, you tend never to get it again. Our adaptive immune system is the principle behind vaccination. People can get vaccinated against measles as infants, for example, setting in place a “memory” of the virus, and retain immunity for life.

Why can you get a few shots against something like mumps as a kid and forget about it, whereas there’s a new flu vaccine out every year? Because the influenza virus, in the words of the World Health Organization, is a “Master of Metamorphosis.”²⁰³

Influenza viruses are like Hannibal Lecter. They bud from infected cells and cloak themselves in a stolen swatch of the cell’s own membrane, wrapping itself in our cells’ own skin as a disguise. The virus can’t completely hide, though. Out of necessity it must poke neuraminidase and hemagglutinin spikes through the membrane to clear through mucus and bind to new cells. These are the primary targets, then, that our antibodies go after. So that it doesn’t go the way of chicken pox, the influenza virus’s only chance for reinfection is to stay one step ahead of our antibodies by presenting an ever-moving target.

The World Health Organization describes influenza viruses as “sloppy, capricious, and promiscuous.”²⁰⁴ Sloppy, because an RNA virus thrives on mutation. The human genetic code is billions of DNA letters long. Every time one of our cells divides, each of those letters has to be painstakingly copied to provide each progenitor cell with the identical genetic complement. This is a tightly controlled process to prevent the accumulation of mutations (errors) in our genetic code. RNA replication is different.

RNA viruses have no spell-checker, no proofreading mechanism.²⁰⁵ A virus like influenza is no perfectionist. When RNA viruses make copies of their genomes, they “intentionally” include mistakes, so odds are that each new virus is unique.²⁰⁶ Once a cell is infected and its molecular machinery pirated, it starts spewing out millions of viral progeny, each a bit different from the next, a population of viruses known in the scientific world as a “mutant swarm.”²⁰⁷ Essentially, every virus is a mutant.²⁰⁸

Most of the new viruses are so mutated, so crippled that they won’t survive to reproduce, but thousands of the fittest will.²⁰⁹ This represents millennia of evolution by the hour. This is how other RNA viruses like HIV and hepatitis C can exist for years within the same individual; they are constantly changing, constantly evading the immune system.²¹⁰ One of the reasons we haven’t been able to come up with a vaccine against HIV is that it mutates so rapidly²¹¹ —and the influenza virus mutates even faster.²¹² This is why it’s so hard for our immune system to get a handle on influenza. By the time we’ve mounted an effective antibody response, the virus has changed appearances ever so slightly, a process called genetic “drift.” That’s why influenza can come back year after year.

Professor Kennedy Shortridge, the virologist who first identified H5N1 in Hong Kong’s chickens, describes influenza as being caused by an “unintelligent, unstable virus.”²¹³ A fellow colleague put it bluntly: “Flu’s not clever. Forget this idea that the virus is clever. The virus is clumsy. It makes lots of mistakes when it’s copying itself, the ones that have an advantage get selected, and that’s why it’s successful.”²¹⁴ No other human respiratory virus has this kind of mutation rate.²¹⁵ It’s like the worst of all worlds: respiratory spread of a deadly virus with a high mutation rate. Scientists fear H5N1 could become like an airborne HIV—half the mortality rate of HIV, but able to kill within days.