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.
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."
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."
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.
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.
