Missing Microbes: How the Overuse of Antibiotics Is Fueling Our Modern Plagues (7 page)

But before we look at those consequences, let’s get to know what we are fighting. Other than potentially harming us, pathogens differ in many ways, for example, their biological nature—are they bacteria or viruses? Do they produce a toxin that injures our cells while they live offshore, in the middle of the gastrointestinal tract, like a battleship lobbing in shells along the coastline? Or are they like the marines, aggressively coming onshore and inflicting damage that way?

It’s tempting to think of pathogens as intrinsically evil, but they’re not. Just like Yellowstone’s wolves, they are predators. More often than not, by pursuing their own survival, pathogens inflict terrific damage on the hosts they inhabit. Sometimes the damage is accidental, the pathogen’s cost of doing business. But for pathogens that are well adapted to their host, the damage serves a purpose. For example, the bacteria that cause tuberculosis make people cough, thereby spreading themselves around and infecting other people. Similarly, the rabies virus attacks the part of the brain involved with aggressive biting behavior and is spread via saliva in infected animals.

David Quammen, in his book
Spillover
, about emerging infectious diseases, notes that we think of predators as big beasts that eat their prey from the outside, whereas pathogens are small beasts that eat their prey from within. It’s an apt description.

The Inuit believe that “the wolves keep the caribou healthy.” A healthy caribou can easily ward off wolves, whereas wolves spot weaker members of the herd, rush in, and tear them apart for dinner. They thin the herd. It’s the same with pathogens. Seven billion people live in today’s world, often existing in squalid, crowded conditions. Malnourished, weak, and often without access to modern drugs, impoverished humans can be easy prey for the pernicious pathogens discussed in this chapter. I’m not saying that thinning the human herd is a good thing. Just that it has always happened and assuredly will happen again.

There are pathogens that simply get under your skin through cuts and scrapes. When a wound is not properly cleaned, you can get an infection, but it is treatable: if mild, just with cleansing, a band-aid, and a kiss; if more severe, then with deep cleansing. Sometimes antibiotics are needed. These cases are pure accidents. The pathogens almost never spread to another person.

Organisms that ordinarily are not very pathogenic (disease-causing) can evolve extraordinary levels of virulence and can also kill robust, healthy individuals in a very short time. Most of us carry
E. coli
in our intestine, and most strains don’t harm us at all. But in 2011 there was a huge outbreak of
E. coli
infections in Germany when people ate contaminated sprouts. At least two
E. coli
strains exchanged genetic material, producing an extremely virulent organism that infected more than four thousand people, damaged the kidneys of more than eight hundred of them, some permanently, and killed fifty.

Communicable diseases are caused by microrganisms that colonize your body, multiply out of control, and make you ill. They can be viruses that cause the flu, bacteria that cause whooping cough, fungi that grow in the lining of your mouth, or a variety of free-living single-cell organisms called protists, such as a nasty amoeba that causes dysentery and bloody diarrhea. More than fourteen hundred human pathogens are currently recognized. They can be high or low grade. The rickettsia that caused spotted fever in the previously healthy young boy is a high-grade pathogen, whereas the kinds of organisms affecting people with chronic lung diseases can be low grade, meaning they are less virulent. They cause illness when a person is compromised and are less likely to make a perfectly healthy person ill.

Ultimately, all communicable disease–causing microbes come to us from our primate cousins, from our domesticated animals, and in other ways that are increasingly dangerous, including from wild animals. Some “jumped” from animals to humans so far back in the past that we can’t be sure of their origins. But other diseases can be traced: plague from the fleas that live on rodents, rabies from bats, influenza from birds, Lyme disease also from rodents but now via ticks. Some of the deadliest pathogens are rogue viruses that have emerged much more recently: Ebola, SARS, Hantavirus, Marburg virus, swine and bird flu. They are virtually impossible to eradicate because we humans can come into contact with the animals in which they live in all sorts of ways. When intermediate vectors like mosquitoes help transmit disease, as with malaria, the picture gets especially complicated.

Some of the most successful human pathogens no longer need their original animal reservoirs. Somewhere along the line, the smallpox, polio, and measles viruses evolved to specialize in humans; they affect us exclusively (and thus are vulnerable to elimination once and for all from humans—like smallpox). But the 800-pound gorilla of recent pathogens, HIV, which jumped to humans from chimpanzees, is now transmitted from person to person through sexual intercourse and contaminated blood. From occasional chance events, over 100 million people are now infected. As I will discuss in chapter 15, I am concerned that we are creating the conditions favorable for the spread of other pandemic microbes by the combination of easier global travel and the lowering of our defenses.

*   *   *

For the vast majority of human history and prehistory, the pathogens behind the world’s great epidemic diseases—smallpox, measles, influenza, plague, polio, cholera, typhoid, scarlet fever, and diphtheria—did us no harm. They did not kill us. The reason has to do with population size. When our ancestors were hunter-gatherers in central Africa, they lived in small groups—maybe thirty to sixty individuals—and were widely dispersed across the vast savanna. They lived this way for about 2 million years before
Homo sapiens
arose, about two hundred thousand years ago. Our existence in civilizations going back about eight or ten thousand years is just a punctuation point on our enormous prehistory. That long period shaped who we are today.

Our ancestors were self-sufficient. When times were plentiful, males brought home enough game to nourish the group; females foraged for fruits, nuts, and plants. But when food was scarce, people suffered. Hunters exhausted themselves trying to find game. Malnourished women stopped menstruating or lactating. Worst of all, when severe droughts persisted, entire groups died out, leaving no trace. Hyenas and vultures picked their bones clean.

But from our modern perspective, this precarious existence had one good thing going for it: there were no epidemics. Our ancestors suffered from common infections, such as parasitic worms and yaws, which are chronic, nonfatal disorders. There were no epidemic diseases because these tiny bands were totally isolated, with no neighbors to bring harmful bacteria or viruses into their communities. If by happenstance a lone individual with a contagious disease stumbled into their settlement, the outcome could go one of several ways: nothing happened, everyone became ill and died, or a few became ill and the rest became immune. But after that, the pathogen had nowhere to go. There were no new hosts to infect. It was marooned and died out.

But the hunter-gatherers did have to contend with latency. Eons ago, tuberculosis and several other well-known pathogens developed this strategy—latency—which permits them to infect one generation, lie low, and then infect later generations, thus avoiding the problem of what happens when no new susceptible hosts are around.

Another example of latency occurs with chickenpox. If, like many children, you breathed in the varicella-zoster virus, you would have soon developed a fever and then broken out in a rash, with blisters all over your body. After a few days, the rash would have faded. In two weeks, you’d be back to normal. With rare exceptions, a child who contracts chickenpox develops lifelong immunity to varicella-zoster. That is the end of the story, or so it might seem. But the virus is clever. It sequesters itself in nerve cells along the spine and in equivalent locations in the head. The virus endures like this for decades, silently, stealthily, causing no discomfort.

Until one day, when you are in your sixties, seventies, or eighties, you might feel a tingling sensation under a rib on one side of your body. The next day, you notice a rash following the contour of your rib. Upon close inspection, you see that the rash has blisters similar to those you had as a child with chickenpox, only this time it is localized. You now have the shingles, which doctors call
herpes zoster
.

In general, the older you are, the more likely you are to get shingles. For decades your immune system is able to hold the virus in check, but as your system weakens with age, the virus is no longer suppressed and out it pops—as zoster. And when the zoster blisters break from their elderly host, the virus spills out into the air, where it can infect a child who has not yet acquired immunity.

And so the cycle repeats. In this way, varicella-zoster can skip entire generations. Although there may be no cases of acute infections in a small community for decades, the virus can “come alive” at any point and then infect a whole new group of susceptible people who were born in the years since the last active transmission. The virus, well adapted to humans, has two opportunities for transmission: from a child with chickenpox or from an aged relative who had chickenpox long before and now has shingles. Contagious, latent, contagious—this is a strategy that optimized success during the long period when our ancestors lived in small bands as hunter-gatherers in the African savannahs.

The bacterium that causes tuberculosis is transmitted similarly, both acutely and after reactivation of a latent infection, usually in an elderly person, which optimized its survival in the small isolated populations that dominated our prehistory. But when human populations later expanded, tuberculosis took off like a rocket.

Small populations are now the exception. About ten thousand years ago, the invention of agriculture made food supplies secure. Populations soared. Trade flourished. Towns grew into cities and crowding was commonplace. And that’s when epidemic diseases began to take off.

Measles is the best-known example for illustrating the working of these so-called crowd diseases. Epidemics often occur in waves and spread quickly from one person to the next until nearly everyone is exposed. In a short time, you survive or die. In the case of measles, survivors developed antibodies and remained immune for the rest of their lives.

Caused by the rubeola virus, measles is the most infectious disease known to humankind, with an infection rate greater than 95 percent. In contrast, a new strain of influenza might infect from one-third to one-half of those exposed for the first time. When I worked in Africa as a student, I saw many children with measles. Typically, they had high fevers, inflamed throats, red eyes, and hacking coughs. The cough, which yielded virus-laden aerosols, was very effective in spreading the disease. Any child not previously exposed to the virus became infected immediately. After a week or so of cough and runny nose, a characteristic rash appeared at the back of the ears and then spread to the rest of the body: measles. Now kids across the developed world are vaccinated, but Africa and other less-developed parts of the world have been slow in catching up. In 2011, there were 158,000 measles deaths globally; 432 people, mostly kids, died from measles every day—18 deaths every hour.

For the measles virus to survive, it must encounter a new susceptible person every week or two. Like a Ponzi scheme, it requires new victims with startling regularity. In fact, measles can be sustained only if there is a contiguous human population of 500,000 people. In such circumstances, a 3 percent birth rate provides 15,000 newly susceptible children each year, guaranteeing measles transmission year after year. But we have had contiguous populations of 500,000 for only about 10,000 years, and thus the epidemics they enable. So measles might have jumped from animals to humans many times before in prehistory, but without sufficient population size it died out.

For instance, many islands, like the Faroes in the North Atlantic, were free of measles for decades at a time. But when a ship brought in an infected person, as one did in 1846, the measles virus quickly spread from person to person. Essentially everyone got the disease. A similar outbreak occurred in Hawaii in the mid-eighteenth century, when measles was introduced by a sailor. People burning with fever went down to the ocean to cool off. But it didn’t help; when the epidemic was over, one in five people had died. The virus died out, only to return by ship many years later.

The rise of cities brought other dilemmas. We had to store food and that attracted hungry pests and their parasites. Scavengers like rats came to visit our grain bins and trash heaps. Thus arose bubonic plague, transmitted by fleas on rats and caused by the bacterium
Yersinia pestis
. The so-called Black Death erupted in Europe in 1347 and within a decade wiped out a quarter to a third of the population. Once introduced, it could even spread without rats, as infected fleas hopped from person to person, and people with plague pneumonia coughed on others.

In 1993 plague broke out in Kinshasa, Zaire. Years of war and corruption caused the government to print money. As a result, there was hyperinflation. People bought whatever they could today, because tomorrow it would cost more. So they stored a lot of grain. This hopeful act brought rats and the plague they carried into many homes.

The Industrial Revolution caused populations to balloon, and many diseases transmitted from person to person worsened. Scarlet fever caused by streptococcus, diphtheria, typhoid fever, and tuberculosis all ravaged the crowded cities. In 1900, tuberculosis was the leading cause of death in the United States. Diarrheal diseases, spread by the contamination of drinking water with sewage, sickened the growing numbers of susceptible people. Twenty percent of children did not survive to age five because of diarrheal diseases, whooping cough, diphtheria, scarlet fever, and other epidemic diseases.