The World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) are tracking an apparently new form of pneumonia they have designated as Severe Acute Respiratory Syndrome (SARS). So far around 500 people are known to have been affected, nine of whom have died. Despite the low number, the WHO has called a Global Outbreak Alert and researchers are trying to find out what could be causing the disease. The CDC is distributing cards outlining the disease symptoms to all passengers arriving on flights from Hong Kong, Vietnam, and certain parts of China.

The speed with which this activity has been organized and coordinated offers remarkable testimony to how truly interconnected our world is and how sensitive our health monitoring systems are. SARS has been identified and a research effort launched after it has affected only 500 people out of the world's total population of 6.2 billion. SARS is contagious, but for now, it appears to take close physical contact with an infected person for someone to contract it.

Medical monitoring agencies have been anxiously on the lookout for an outbreak of a pandemic disease like the Black Death or the Spanish flu. The Black Death of 1347-1351 killed 25 million people, one-third of the population of Europe. The Spanish flu of 1918 killed between 20 and 40 million people worldwide. Medical monitors caught SARS so quickly because they are constantly on the alert for new strains of influenza arising from China. China is a hotbed for new flu viruses because people, domestic fowl, and pigs, which all harbor a wide variety of flu viruses, live in close proximity. This allows flu viruses to combine and mutate rapidly into new, possibly virulent strains.

Another bug is also worrying doctors nowadays: methicillin-resistant Staphylococcus aureus (MRSA). Outbreaks of this drug-resistant staph bug have occurred in Southern California. MRSA is a variety of staph that has evolved drug resistance inside hospitals, where it is regularly exposed to antibiotics related to penicillin. Until recently, MRSA has chiefly been a danger to people with weakened immune systems. Most MRSA infections were acquired in hospitals, but now it has broken out and is infecting people in the general population.

Infectious diseases like SARS and MRSA raise the question of how to balance public health and individual liberty. Prior to the advent of modern medicine, coercion was often used to stop the spread of disease. During the Black Death, city officials would often wall up houses in which an infected person was found, trapping healthy family members with them. In the wake of the Black Death, Venice invented quarantine, designed to protect the city from plagues by requiring ships to anchor offshore for 40 days before their crews were allowed to land. It may well have been in the individual interest of each sailor to get off his ship as soon as possible, especially if some shipmates were infected, but it was in the community's interest to protect itself against infectious disease.

Today the United States still runs a quarantine service and has the power to detain persons suspected of carrying dangerous communicable diseases. Throughout history people resisted quarantine, but hopefully today people are more likely to go along with it because they realize that they will have access to the benefits of modern medicine.

Quarantine may become more relevant if a serious bioterror attack occurs or if a highly infectious disease for which there is no good treatment emerges naturally. Protecting the majority from death and disease can and should override a person's right to freedom of movement.

The spread of MRSA, or any other antibiotic resistant pathogen, is a subtle example of the tragedy of the commons. The tragedy, popularized by Garrett Hardin in a famous article of that title, is that absent individual property rights in any common, everyone's incentive is to overuse the resources available there. And MRSA probably arose because of the overuse and misuse of antibiotics, since everyone's individual incentive is to overuse them, not considering the ill effects this can have for the whole human population.

The staphylococcus bacteria have been repeatedly challenged by antibiotics and have evolved resistance to them in order to survive, making the antibiotics useless for people who become infected later. A sick person goes to his doctor with an infection. They pressure the doctor for some medicine and she prescribes an antibiotic, even though the illness is most likely a viral infection that does not respond to antibiotics. Or the patient is given pills that do work against their illness, but they fail to take all of them, leaving some bacteria behind which are now stronger. In these cases, what is good for an individual–using an antibiotic on the off chance that it will cure their diseases—makes that medication less useful for other sick people later. Alas, while the standard tragedy of the commons problem can be solved by establishing and respecting individual property rights, it is hard to see how to apply a property-rights solution to the problem of individuals overusing antibiotics in a way that potentially threatens the whole human race.

This problem may be resolved as faster in-office methods for diagnosing diseases are developed. Then physicians will know for sure whether antibiotics are appropriate treatments or not. Until then, physicians should deny individual patients antibiotics if their best medical judgment indicates that they will be ineffective.

Until somehow we can all be held responsible for our own microbes, managing the medical commons by sometimes coercive means will be necessary.