Nature being capricious and taking pleasure in creating and producing a continuous succession of lives and forms because she knows that they serve to increase her terrestrial substance, is more ready and swift in her creating than time is in destroying, and therefore she has ordained that many animals shall serve as food one for the other; and as this does not satisfy her desire she sends forth frequently certain noisome and pestilential vapours and continual plagues upon the vast accumulations and herds of animals and especially upon human beings who increase very rapidly because other animals do not feed upon them.
—LEONARDO DA VINCI
To move from where it is to the next available host, a microbe has to have a way of getting there. This is what we call means of transmission. Over the millennia, various pathogens have evolved different means of transmission, which are a prime factor in how much we need to worry about them.
The four categories enumerated in the chapter title do not represent a complete list, but rather the principal concepts we need to understand regarding disease spread.
Bats are a type of disease reservoir, which means a place where pathogens maintain themselves. For example, we believe, but have not yet definitely proved, that Marburg filovirus—a close cousin of Ebola—resides in fruit bats that live in locations such as Kitum Cave in Kenya’s Mount Elgon National Park. The virus is excreted in the bats’ guano and migrates from there. It’s important to note that reservoirs need not be animals, or even alive. A reservoir can be a plant, a body of water, or any other host in which the pathogen can multiply and survive while it waits for its next spread. As we have seen with Marburg and Ebola, trying to discover or figure out the reservoir can be one of the great whodunit factors for a disease detective.
The mosquito is what is known as a vector: an arthropod that carries and transmits a pathogen into another host. Mosquitoes are the kings of vectors, our ultimate foe. In addition to prevention of illness through vaccines or other antibiotics, vector control is crucial in halting the spread of disease through mosquitoes and other insects. We will deal with this in depth in chapter 14.
Back in the 1400s, when mosquitoes accompanied mariners to the New World and on other voyages that took months or years, the mosquitoes aboard ships would die out before they could infect immunologically naïve populations. It took humans to do that. Today, a rat or mouse would most likely be noticed aboard a commercial airliner and dealt with before passengers embarked. But a mosquito can hitch a ride anywhere with virtual invisibility.
Lungs, which we all need to survive, are the scariest method of transmission, because through this method we can get sick simply from breathing—specifically, breathing in someone else’s contaminated air. The 1918 influenza outbreak, which we have noted was the deadliest pandemic of the modern era, was an airborne transmission, as are all influenza strains. So-called respiratory-transmitted infections are the most likely candidates for quick spread because all they need is for their hosts to breathe.
Then there is the entire category of sexually transmitted infections, in which bodily fluids are exchanged between sexual partners. This has always been a touchy subject for public health authorities because people don’t like to talk about it and it is difficult to obtain honest reporting and good statistics. Despite the fact that we are all here as the result of a sex act, meaningful discussion remains one of the great societal taboos. With sexually transmitted infections, epidemiology must venture far into the realm of sociology. When it arrives there, what we tend to discover—or relearn—is how difficult it is to get people to alter their habits, and that, in too many instances, women are denied agency over their own sexual destiny.
Syphilis, an age-old scourge caused by the Treponema pallidum bacteria, is one of those ailments no group wanted to claim and all groups wanted to blame on some other. After an invasion by the French in the late 1400s, Neapolitans labeled it “the French disease.” The French, on the other hand, called it “the disease of Naples.” The Russians called it “the Polish disease,” while the Polish and Persians called it “the Turkish disease.” The Turks called it “the Christian disease,” the Tahitians called it “the British disease,” the Indians called it “the Portuguese disease,” the Japanese called it “the Chinese pox,” and so on and so on. A similar worldwide paranoia greeted the advent of HIV/AIDS, which is one of the reasons Jim Curran at the CDC was so insistent that the world scientific community quickly adopt a name that would be completely neutral and the same in every language.
While many of us came of age during the so-called sexual revolution of the 1960s and succeeding decades, we should remember that there was only a narrow window of history in which sex couldn’t kill you: between the widespread availability of sulfa drugs and antibiotics in the 1940s that combatted the bacterial STDs and the coming of AIDS in the early 1980s. And yes, we do now have drug cocktails that keep the levels of HIV under control, but AIDS is still a worldwide killer in much of the poor and developing world, where populations don’t have access to modern medicines. And lest we get too complacent about syphilis, gonorrhea, and the other sexually transmitted pathogens, as we will see later in this book, the future effectiveness of antibiotics is a very shaky proposition. All of this is to say that our common enemy never gives up the fight.
Another aspect of disease transmission by penis that we cannot ignore is rape as a weapon of war. All decent people are appalled by crimes of sexual assault, and horrified when one of them results in a sexually transmitted disease. But throughout history, rape has also been used as a means to terrorize and help conquer an enemy’s civilian population, and today we are seeing it employed on a strategic basis in conflicts in Africa and the Middle East. Suffice it to say that every rapist is a craven and unredeemable collaborator with humankind’s common enemy and guilty of the most damning charge it is possible to bring against a human being: a crime against humanity.
A complex web of factors determines which pathogens can kill us, hurt us, or merely inconvenience us. At the heart of this web is a single critical consideration: How does the microbe get transmitted? In our disease-control business, transmission is defined as any mechanism by which the microbe is spread through the environment or to another human or animal. These mechanisms may include direct body contact with a human or animal; breathing air that was just exhaled from another person or animal, an aerosol purposefully sprayed into the air, or a mist from a nearby building’s cooling tower; consuming food or water; physical contact with a surface such as a door handle; a mosquito or tick bite; a blood transfusion; or contact with blood on a previously used or contaminated needle.
While all of these mechanisms are significant spreaders of specific diseases, the ability to transmit a microbe by merely breathing it into our lungs is the most dangerous. We call this airborne transmission. In the real estate business, they say, it’s “location, location, location.” In public health, it’s “airborne, airborne, airborne.”
The potential for airborne transmission of a virus was demonstrated in stark clarity with a measles outbreak investigation I led in Minnesota in 1991. The outbreak occurred in conjunction with the Special Olympics and an infected twelve-year-old male track-and-field athlete from Argentina. He was in the highly infectious early stage of illness when he stood for several hours near home plate during the opening night ceremony in the enclosed Hubert H. Humphrey Metrodome. Other competitors, game officials, and support staff came down with measles after exposure to the young athlete. Two of the subsequent cases were Minnesota residents who did not know each other and did not attend any other Special Olympics event beyond opening night. But both sat in the same upper-deck section, more than 400 feet from home plate. Stadium airflow circulation data for that night supported the conclusion that air from where the athlete entered the stadium or from where he stood at home plate would have been pushed toward the two spectators who developed measles.
The most notorious of these airborne-transmitted diseases is influenza, and while we classify flu strains by subgroups of two of its surface proteins—hemagglutinin and neuraminidase, HA and NA—for our purposes here we’ll divide flu viruses into two groups. The first is seasonal flu: the kind that makes you feel miserable, fills hospitals most winters, causes widespread absenteeism from schools and workplaces, and kills between 3,000 and 49,000 people each year in the United States. The other is pandemic influenza, which occurs when a new flu virus emerges out of the animal world through mutation or reassortment so that it can infect and be transmitted by humans. Generally, seasonal flu is a remnant of a strain of the flu virus that once caused a pandemic.
Throughout history, influenza and its ability to quickly kill many millions during a global pandemic has earned it the status of king of infectious diseases. An infected human can efficiently transmit the flu virus to people around him, and unlike someone infected with Ebola, say, he can do this even before he shows signs of being sick. All that is required is to breathe the contaminated air just exhaled or coughed from the lungs of an infected person. Imagine that person on a plane or subway car or at a shopping mall or sporting event where we all share one big common bucket of air. And remember how many people fly around the world each day as we consider how fast a disease like influenza may spread across the globe. Unfortunately, I am certain we are actually more vulnerable worldwide to an influenza pandemic today than we have been at any time over the past five centuries.
Airborne transmission is also a major concern with regard to the use of microbes for terrorism attacks. We now know that highly infectious Bacillus anthracis spores, the cause of anthrax, can travel many miles in the air when released in a simple, relatively easy-to-prepare powder from an airplane routinely used in agricultural crop spraying or mosquito control. Breathing in just a few of these spores is enough to cause a life-threatening reaction.
The next most concerning category of disease transmission is really a toss-up. As long as AIDS cases continue to increase in numbers around the world each year as a result of direct-contact spread, namely, through sex or through birth to an HIV-infected mother who is not receiving appropriate HIV drug treatment, this mode of disease spread is of critical public health importance. I don’t include HIV transmission from sharing contaminated needles here, as it is technically classified as indirect transmission. It, too, remains an important part of the HIV risk picture, but direct-contact spread is still the most critical aspect of HIV today. Yet while this disease remains a high public health priority because of its international morbidity and mortality, particularly in Central Africa, the development and availability of the drugs that have made it a “livable” chronic condition have taken away its emergency or crisis profile in wealthier countries.
The second transmission category in the toss-up is vector-borne diseases—those transmitted by mosquitoes, ticks, and flies. We have now moved many species of mosquitoes that can transmit any number of infectious diseases to humans and animals around the world inside aircraft and cargo ships. When mosquitoes originally native only to Southeast Asia are transported to the Americas inside tires in the holds of cargo ships, they proliferate quickly in their new homeland. Never before in the history of humankind has there been the current extensive number of species of microbe-carrying mosquitoes on every continent except Antarctica. As a result, in just the past fifteen years we have witnessed the major global spread of diseases like dengue fever, West Nile virus, chikungunya, and Zika. And we still have to consider the reemergence of yellow fever and highly drug-resistant malaria. This disease transmission category also does not bode well for us as it relates to global climate change. A warmer world presents us with the potential for less overall precipitation in some regions. But when it does rain, it will be in monsoon-level amounts. This means that disease-causing mosquitoes will be sharing even more territory with large human populations.
The final transmission category we are calling “current world conditions”: an amalgam of factors within three very different, yet highly microbe-rich environments. First is the exploding human population in the megacities of the developing world and the packed, horrible conditions in which the unfortunate residents live. Second is the human contact with animals in the rain forests of Asia, South America, and Africa, the ultimate fertile grounds for new and dangerous human pathogens, which are now spilling out into the inhabited world. Third is the high-intensity animal-production facilities around the globe that represent millions of new, living, animal “test tubes” for microbes, born each day.
Why were we surprised that Ebola virus, a disease that to date is spread by direct contact with contaminated body fluids, moved as quickly and efficiently as it did in the villages and slums of the three impacted West African countries? Why are we surprised by the unprecedented increases in avian influenza viruses—the precursors for a human pandemic influenza strain—associated with exploding global poultry production? Why were we surprised by the rapid spread of Zika virus throughout the Americas when the Aedes aegypti mosquito, the vector for this disease, is now widespread within this area?
If there is a lesson here, it is that we have to think seriously about these things. And we haven’t been.