Why can’t we eradicate all diseases and How was Smallpox eradicated in 1979?

We often speak of the possibility and desirability of eradicating diseases, and it does seem that it should be possible to eradicate at least those whose causes we know and for which we have good and effective treatments.

This would seem to apply to most of the sexually transmitted diseases. But in all of history, only one disease, smallpox, has ever been eradicated, and that happened only after a vaccine for the illness had been in use for 181 years and even then only after we had designed and carried out an elaborate and expensive program of vaccination that required worldwide cooperation through the United Nations.

People sometimes say, either wistfully or angrily, something like this: “If we could just stop spending money on weapons, we could eliminate . . .” or “if only we had the political will to do so, we could eradicate . . .” and then they fill in the name of some terrible illness. Unfortunately, it isn’t that simple. True, a handful of infectious diseases undoubtedly can be eliminated if we devote the money and international effort to doing so.

With a few others it may be possible to eliminate some of the worst consequences of infection. But there are some diseases that cannot be eliminated, no matter how earnest our efforts and how fervent our desire to do so, because the nature of the microbes that cause them makes this impossible.

Smallpox is in many ways a perfect candidate for elimination, in fact, among infectious diseases, it is surely the easiest to extinguish. It is relatively noncontagious, usually requiring very close contact to be transmitted. The disease is easy to diagnose by its characteristic skin lesions or by antigen detection.

It only infects humans, you can’t catch it from an animal, wild or domestic. Immunity develops after one infection, and there is no carrier state, or period in which a person can be infectious but asymptomatic.

We have had a vaccine for it for more than 200 years, and have had ample time to perfect that vaccine so that it is virtually 100 percent effective and 100 percent safe. The vaccine is inexpensive and highly resistant to temperature changes, chemicals, and drying, so it can be shipped over long distances without refrigeration or other special requirements. It requires only one dose and can be given to infants at birth. Several decades can pass before a booster vaccine is required.

The smallpox virus existed in only two strains, neither of them resistant to the vaccine, probably partly because the illness has never been treated, only prevented. In other words, smallpox was in many ways ripe for elimination. And even then it took us almost 200 years after we knew exactly how to prevent the disease to actually eliminate the germ.

Now think about a microbe that isn’t as cooperative as the smallpox virus. Think, for example, of the bacterium that causes tuberculosis. There is a vaccine for TB, rarely used in the United States but sometimes used, mostly on children, in other countries. The problem with it is that it isn’t very effective, you can still get TB even if you’ve had the vaccine.

Obviously, for anyone who already has TB, the vaccine is completely useless. So that leaves treatment. Treatment of TB requires repeated doses of medicines over a period of six months or more (isoniazid, rifampin, pyrazinamide, ethambutol, streptomycin, you have to take more than one drug to prevent the emergence of resistant mutations in people in whom the bacterium is rapidly reproducing). If you skip doses, the treatment is not only ineffective, but actually makes the disease worse by creating strains of bacteria resistant to the medicines. You can’t trust people to take their medicine, they forget, they misplace it, they just don’t feel like it today, they have a million excuses, so you have to actually go out and find them where they live, give them their medicine, and watch them swallow it.

This is exactly what health care workers do in the United States, in a program called DOT (directly observed therapy), and of course there aren’t enough people to do it, and even when there are enough people to do it they don’t always succeed. If the person is actually infectious, that is, if the TB is in the lungs or throat, he has to be isolated from other people while he’s treated. And if he’s sick and bedridden, he has to be isolated in a hospital room, usually for several weeks, not just an ordinary hospital room, but a specially designed room that requires expensive and elaborate equipment, including a ventilation system that creates negative air pressure inside the room (the bacillus is airborne) and vents it through a series of filters to the outside.

So TB treatment can be very expensive and is only partially successful in the United States, where a highly sophisticated and efficient health care infrastructure is in place. Now imagine what it would mean to establish a TB treatment program in a Third World country where there are hardly any health care workers, no clinics or hospitals worthy of the name, severely limited communication and transportation facilities, a population widely dispersed in rural areas, rampant HIV infection that makes TB infection much more common and severe, and where, on top of all that, there’s a civil war going on with half the population trying to kill the other half, not cure them. And we haven’t even discussed the side effects of the medicines, which can include everything from vomiting to hearing loss and at least one of which (isoniazid) can cause fatal liver disease if you drink alcohol while you’re taking it.

Even with all of these barriers, it is not completely out of the question that tuberculosis might be eliminated, at least in some places. Infectious TB is relatively easy to diagnose by a simple laboratory test. The illness is treatable and curable, and treatments costing as little as $18 are available for simple cases. While asymptomatic people can carry the disease, they can nevertheless be identified by skin testing, and reactivation TB can often be prevented by administering isoniazid to people found to be at risk. The only other reservoir of the bacterium is in dairy cattle, and this form is rarely associated with human infection. Directly observed treatment-short course (DOTS) is highly effective, even though it is difficult to administer. Eliminating it globally may present a challenge currently too difficult to overcome, but extinguishing it in a highly developed country is conceivable even despite the staggering barriers we’ve just described.

Yet there are diseases that are even harder to eliminate than tuberculosis. In fact, there are diseases, quite common ones, that cannot be eradicated no matter what we do, even though they are well understood and vaccine-preventable. Pertussis, or whooping cough, is a good example. Pertussis is caused by a bacterium called Bordetella pertussis, which was first isolated almost 100 years ago. It causes paroxysms of coughing, often followed by vomiting, and its complications include hypoxia (lack of oxygen), pneumonia, seizures, encephalopathy (brain disease), and malnutrition.

Infants and young children are most at risk, and pertussis is deadly. Worldwide, it kills approximately 400,000 people per year. It is very contagious, when one person in a family gets it, there is a 90 percent likelihood that the rest of the susceptible people in the household will, too. There is a vaccine for pertussis, and a large majority of infants and children up to six years old in the United States get it. But the vaccine has certain serious drawbacks. While it is effective in preventing severe disease, it may not prevent acquisition of the infection, and it doesn’t prevent mild disease.

Even mild disease is highly contagious. The vaccine requires multiple doses, so the delivery system for it has to be highly organized and requires considerable cooperation from patients and their caregivers. After you’ve been vaccinated, you’re immune only for a limited period of time, and if they don’t get boosters regularly, older children, adolescents, and adults will become susceptible, and the vaccine is not licensed for people over the age of six years.

Pertussis is difficult to diagnose, especially in the phase in which it is most infectious, so control of outbreaks by quarantine and other measures is difficult or impossible. To prevent transmission, you have to make more than 90 percent of the population immune, which isn’t attainable even if you vaccinate 100 percent of them. The vaccine is at best 90 percent effective. In fact, the latest reports from the CDC indicate that pertussis is actually increasing among infants, providing a steady reservoir of infection. Eradication of pertussis is simply not feasible.

Among the STDs we’ve discussed, hepatitis B is realistically a candidate for eradication. There is a very effective vaccine that can be given at birth, and while most chronically infected people remain infected for life, their level of infectivity declines over time. Humans are the only known host for the virus. If we can sustain elimination of transmission long enough for people with chronic infections to die, we could conceivably make the virus extinct. The key is preventing perinatal and infant disease by universal infant vaccination. Young children, adolescents, and adults can be given catch-up vaccinations. Identifying infants who need this postexposure treatment is not difficult in industrialized countries, although it may present challenges in nations that do not have substantial health care infrastructures.

Congenital syphilis is probably more difficult and expensive to eradicate, but it is within the realm of possibility because it can be prevented if infected pregnant women are treated with penicillin. There are no nonhuman hosts of the syphilis bacterium, tests for the disease are very accurate, and early syphilis can be treated with one injection of an inexpensive medicine, all factors that suggest eradication is possible.

Eradication of any disease is easier in industrialized countries with good health care systems than it is in other parts of the world, and this certainly holds for congenital syphilis. Elimination of congenital syphilis will require that syphilis rates be reduced, which the continuing national campaign against the disease has significantly done. Between 1990 and 2000, syphilis rates fell 90 percent. Rates among blacks and Hispanics remain considerably higher than among whites, and work with these groups is urgent. In addition, the CDC is concerned with outbreaks of syphilis among gay men. Nevertheless, there is now renewed hope that sexually transmitted syphilis might also be eliminated in the United States.

As for the other STDs, we are going to have to continue trying to encourage people to behave in ways that prevent transmission, but always with the awareness that eradication of these diseases is little more than a hopeful fantasy, and that we will live with these microbes, for better or worse, for the rest of our lives.