Pandemic! 10 of the Deadliest Diseases

The Black Plague, Third Pandemic and Spanish Flu wiped out hundreds of millions; they have nothing on today's worst diseases

What makes a disease deadly in the twenty-first century? Medicine has never been more advanced; our understanding of spread and infection, never more sophisticated. And yet, we may be poised for the largest and most devastating pandemic the human race has ever encountered.

Diseases that could have been effectively eradicated decades ago continue to ravage developing nations. In the wake of natural and manmade disasters, cholera, tuberculosis and the like spread even more easily, aided by tenuous medical infrastructures and close living quarters for refugees. Meanwhile, wealthy nations are no less imperiled, their citizens endangered by a massively consolidated food supply and by antibiotics prescribed so indiscriminately as to potentially destroy their efficacy altogether.

But, if medical advancements may be our undoing, they also pose our only salvation. Launch the gallery here to see 10 of the world's deadliest diseases—the contagious monsters that threaten our very way of life—and to learn how science is holding them at bay.

Salmonella
Salmonella and_ E. coli_
This year's big foodborne threat is killer tomatoes. Two years ago, spinach up and vanished from grocery store shelves around the country. Michael Pollan will be the first to tell you why: "Eighty percent of America's beef is slaughtered by four companies, 75 percent of the precut salads are processed by two and 30 percent of the milk by just one company." The consolidation of the industrial food supply necessarily means that any pathogen which enters the system will have no trouble finding its way to your dinner plate, heedless of global distances. Compounding that problem, we have the issue of antibiotics being administered as a preventative measure in livestock and poultry. Animals are routinely fed these medicines as part of their diet, whether they are sick or not. This indiscriminate use has undoubtedly led to a reduced efficacy of antibiotics in humans. Dr. Arjun Srinivasan, a medical epidemiologist with the CDC, notes that we don't know whether overuse of antibiotics in humans is ultimately worse than overuse in animals, but that "there are those who say, if you look at the absolute amount of antibiotics that are used in animals, [it] vastly outweighs the amount that's used in humans. So therefore, that may actually be a larger component" of the problem.NIH
Yellow Fever Virus
Yellow Fever Virus
The first of two agents on our list spread by the Aedes mosquito, the yellow fever virus wasn't been much of a concern in the latter half of the twentieth century. Malaria control efforts in the 1950s successfully decimated the Aedes population, and with it the occurrence of yellow fever. In the past few decades, however, the mosquito has returned and is ranging much further than previous generations. It's also making its way into urban environments, which it has done in the past—an outbreak nearly wiped Memphis off the map in 1878—but in recent memory, it has been confined to the tropical jungles. The fever gets its name from the jaundice it can cause after a few days of infection. Later comes internal bleeding (it's a hemorrhagic fever like Ebola and Marburg) followed by bloody vomit with the consistency of coffee grounds. What is most worrying about its return to cities is that it achieves a higher mortality rate among dense, unexposed populations—up to 30 percent. Recent outbreaks in Paraguay and the Ivory Coast have health officials racing to vaccinate as quickly as possible. While an effective vaccine exists, there is no treatment and no cure.CDC
Shanghai SARS Alert
Shanghai SARS Alert
Nobody used to pay much mind to the coronaviruses. While the genus is home to two species responsible for the common cold, they haven't received the attention given to other cold-causing viruses because coronaviruses are difficult to grow in a lab environment. That all changed very quickly in 2003 when a new respiratory disease began killing doctors and nurses and showing the potential to spread at pandemic levels was identified as a previously unknown coronavirus. The infection was severe acute respiratory syndrome, or SARS, and it held the world's attention for just under a year before it disappeared in the summer of 2003. The global public health response was a near-unparalleled success. Within weeks, control efforts led by the World Health Organization had identified a totally novel agent, devised a diagnostic test, and instituted plans for quarantine and isolation. It is undoubtedly a result of those efforts that the outbreak was contained before it could reach pandemic levels. And while it is no longer topping watch lists, two questions persist: how did it get to humans and where did it come from? As Dr. Scott Dowell, head of the CDC's Global Disease Detection Program explains, "how it is that one of these animal pathogens acquires the ability to spread efficiently among humans is something that we don't do a very good job explaining or predicting." Coronaviruses are known to mutate rapidly, so there may have been some biological basis to its sudden appearance and virulence, but it was still very much a surprise. Where it currently lies in wait is even more of an unknown. There is evidence the 2003 outbreak originated in a wildlife market in southern China, but the exact species of animal from which it came is still very much in contention.DearTerisa (CC Licensed)
Liver Infected With Ebola
Liver Infected With Ebola
This hemorrhagic fever has gained a special notoriety for being a quick and exceptionally deadly killer. Ebola is known as the fever that kills with a million cuts, because it causes a reaction in the blood that produces microscopic holes in the capillary walls. The patient then bleeds to death internally. Mortality can be as high as 90 percent. It is invariably a headline-grabber when outbreaks strike. But it's not on this list because it's presently a significant threat (it's not). It's here for two reasons. The first has to do with a trait Ebola shares with the SARS coronavirus—its zoonotic host is a mystery. Although the virus has been known to us since the mid-1970s, we are still largely in the dark about what its reservoir is in nature. Even after a comprehensive study of tens of thousands of animals in outbreak regions, no virus was found. That points to the difficulty public health officials face when unknown threats emerge—we have a very hard time tracking some viruses we've known about for decades, so you can imagine the mounting complications when starting from zero. The second reason it's on this list is to place it within the context of the rest of the agents. While it is a ravaging disease, it presents little threat outside of where it appears locally. It is not communicable through the air, and only spreads from person to person; often because of poor hospital conditions in the areas in which it appears. In addition, it presents symptoms very quickly—infected persons are likely to be isolated before getting very far. All the rest of the diseases on this list can spread far and wide, which makes them much more threatening.CDC
Methicillin-Resistant Staphylococcus Aureus
Methicillin-Resistant Staphylococcus Aureus
Methicillin-resistant Staphylococcus aureus,—or MRSA,—is a mutant variant of the common staph infection found in hospitals and nursing homes. What sets it apart from common staph is its resistance to a wide range of commonly used antibiotics. In the late 1990s, it began to appear in people who hadn't been anywhere near a health-care institution. They were struck with what scientists have taken to calling Community-Associated MRSA. The disease appears in places where daily, close contact is the norm: schools, day-care centers, and prisons, for example. If caught early, before it gets into the bloodstream, it is usually treatable with low-grade antibiotics, and its spread can be controlled. It may even be remedied without antibiotics by draining the lesions it raises on the skin. Once it passes that early stage, however, it can become a much more difficult infection to eradicate. MRSA is an important warning sign because doctors are frequently having to use the strongest antibiotics to treat it. We know this to be an effect of antibiotic overuse. The end result is a breed of bacteria against which we have little, if any recourse for a cure. "The challenge that we'll face is that a growing number of bacterial infections will be more and more difficult to treat. The reports are rare, but we're already seeing [cases] of bacteria... where there are no effective antibiotics to treat the infection," says Dr. Srinivasan. Right now, these cases are appearing only in hospitals and only in the most immunocompromised patients, but that was once the case for drug-resistant staph, too. The only real, immediate course of action is education and vigilance about proper antibiotic use, because, as Dr. Srinivasan notes, "our ability to develop new drugs has already been surpassed by the speed with which bacteria are developing resistance." Several institutions have undertaken awareness campaigns, like the CDC's "Get Smart" program and the Infectious Diseases Society of America's "Bad Bugs, No Drugs," both of which have had good success educating both patients and health-care workers.CDC
Dengue Fever
Dengue Fever
Like yellow fever, dengue is hemorrhagic and spread by the Aedes mosquito. Unlike yellow fever, dengue is commonly an urban infection and has no effective vaccine. While infected persons will develop immunity after a bout with the disease, it persists in densely populated locales because it exists in four different strains. Antibodies for each one are useless against the others. Dengue periodically appears in large outbreaks, the most recent of which is in Rio de Janeiro, where an estimated 100,000 people have been infected so far in 2008. Because little can be done about the virus once it infects, efforts to control dengue are focused on controlling the mosquito which carries it. Anyone in this country who has lived in an area in which West Nile virus is a threat is doubtless familiar with the need to remove standing water with vigilance. Whether kicking over discarded tires or emptying plastic cups left in the rain, any disruption of the mosquito's breeding grounds means a reduction in larvae which may survive to become dengue hosts.CDC
Enterovirus 71
Enterovirus 71
Hand, foot and mouth disease is a pretty common childhood illness caused by a variety of viruses generally considered to be benign. Infected kids get a mild fever and spots around their mouths; the whole thing lasts a few weeks. No big deal -- until one of the strains, enterovirus 71, decides to ratchet things up substantially and become highly lethal. Cases of sudden death from EV71 in children have been steadily increasing in Asia since the late 1990s. The most recent outbreak, which began in early May in southern China, has already claimed the lives of nearly 40 children under the age of six, with the number of reported infections climbing into the tens of thousands. It's unclear just how the fatal strain of EV71 manages to kill, but the evidence so far seems to indicate that it travels into the brain stem of a child and from there shuts down the respiratory system. Like many of the viruses on this list, no treatment or vaccine exists. What's worse, there is no reason to think it won't make its way to the U.S. And, as Dr. Dowell explains, "if it does come to the U.S., there's no real reason to think that we would do any better with it than the Chinese in Anhui providence have."MidgleyDJ (GNU Licensed)
Influenza A (Avian Flu)
Influenza A (Avian Flu)
All that stands between us and an influenza pandemic on a scale that could dwarf the Spanish Flu of 1918 is a handful of genetic mutations in a virus known to have a high mutation rate. Presently, the influenza variant known as H5N1—commonly called the avian flu—can only readily move from an infected bird to a human. We have been lucky to limit its spread to no further than any one single family cluster, but that is largely due to the fact that it has yet to acquire the ability to move effectively from human to human. It could simply be a matter of the virus having yet to land in someone already infected with another strain of influenza for H5N1 to pick up the genetic material necessary to make the leap. To give you a little historical perspective of where we may be headed, consider the influenza pandemic of 1918. The overall mortality rate of that flu was considerably higher than the normal annual rate of flu infections, topping out around 2 percent. The H5N1 variant has shown itself to have a mortality rate in the neighborhood of 60%. According to Dr. Dowell, "if there are a few mutations in that virus and it acquires the ability to spread efficiently from person to person, it's hard to imagine historically anything to compare it with."Cynthia Goldsmith
Vibrio Cholerae
Vibrio Cholerae
Within hours of contracting cholera, it is possible to die. The bacteria attach to the wall of the small intestine and immediately begin producing toxic proteins that induce severe, unrelenting diarrhea. Without a very simple remedy of salt and sugar water, a person can dehydrate to the point of dangerously low blood pressure, followed by shock and heart failure. Fortunately, it is relatively easy to control. With proper sanitation and access to clean water, cholera infections are readily kept at bay. When good medical care is available, the mortality rate stays below 1 percent. It's when conditions are bad that cholera thrives. During the Rwandan genocide of 1994, nearly 80 percent of infected, unaccompanied child refugees in Zaire died within the course of a single month. The world is currently in the midst of the longest running cholera pandemic, which has persisted as it has because the strain responsible manages to hide in people without infection more capably than previous variants. Some estimates put the ratio at 50:1 for carriers to actively infected. It has this year appeared as an exceptionally large outbreak in sub-Saharan Africa. It's also been seen in Vietnam and last fall in Iraq.CDC
Extensively Drug Resistant Tuberculosis
Extensively Drug Resistant Tuberculosis
Tuberculosis was once called consumption, because of the way it would overtake a person's being, appearing to consume them from within. Infection causes the victim's eyes to redden and swell, and skin slowly to go pale; the incessant coughing eventually brings up blood. It is an old disease. Its effects have been seen in the bones of prehistoric man. It has managed to insinuate itself in the human population so thoroughly that the World Health Organization estimates one out of every three people on Earth has been exposed to it. For a disease with which we have had such a long and intimate relationship, one would hope we'd have a pretty good handle on things by now. While we have for many years been adeptly developing antibiotics to fight TB, the tuberculosis bacterium has in many ways been more adept at surviving them. Of particular concern are the strains of TB classified as multiple-drug-resistant (MDR-TB); at the top of that list is XDR-TB, or extensively-drug-resistant tuberculosis. XDR-TB is of great concern because it is now resistant to not only the first- and second-line antibiotic agents, but one of the third line as well. The strain is making us reach deep within our well of defenses, and the current concern is that it will soon outpace the remainder of the third line. It has a much higher mortality rate than even MDR-TB, and can be a terribly severe infection. Fortunately, the trade-off for all its virulence is that it does not spread easily among healthy populations, which may be why it is not as widespread as we might expect. Among those with already compromised immune systems, however, it is capable of reaching epidemic proportions.CDC