Are infectious diseases on the rise? Editor's note

James Steckelberg, M.D.

MRSA — the latest in a series of emerging infectious diseases — has caught the attention of health officials and the public alike with evidence that it's becoming an increasing health threat. MRSA joins bird flu, West Nile virus, SARS and HIV as potentially deadly infectious diseases that seem to be continually cropping up. But are there really more infectious diseases now than in the past? The frustrating answer is both yes and no.

Historically speaking, the current pattern of emerging diseases isn't new. From the great plagues that swept through Europe in the Middle Ages to the 1918 influenza pandemic and the polio epidemics that marked the first half of the twentieth century, infectious diseases have always appeared, reappeared and sometimes disappeared over time. On the other hand, certain aspects of modern life make it easier for some diseases to flourish and for researchers to identify them. And the immediacy and ubiquity of modern communications bolsters the perception that we live in unusually threatening times.

Here are some of the factors that influence the spread or emergence of infectious diseases or the way we think about them:

Antibiotic-resistant bacteria

Today, one of the gravest threats to public health is the rise of antibiotic-resistant bacteria. More and more germs that were once easily controlled now defy treatment with even the most powerful medications. In large part, this is due to the misuse and overuse of antibiotics, which has bred a race of superbugs that cause new, more-lethal versions of old diseases.

One example is methicillin-resistant Staphylococcus aureus (MRSA), a type of staph bacteria that are found in hospitals and care facilities worldwide. MRSA can cause massive infections that resist treatment with all but the last-ditch drug vancomycin. And strains of MRSA that are resistant even to vancomycin are starting to turn up in some hospitals.

In the last few years, certain strains of MRSA have been showing up in the wider community with increasing frequency. Although these appear to differ from hospital-acquired strains, they have the potential to cause serious infections and to become just as resistant to antibiotics.

Diagnostic tools

Better diagnostic tools and laboratory techniques, including polymerase chain reaction and gene sequencing, have allowed us to isolate and identify certain diseases that may seem new, but that actually have been around for years. A prime example is Lyme disease — the most common insect-borne infection in the United States. Lyme disease was newly recognized in 1975, but the organism that causes it has since been identified in mice that were collected and archived in the Smithsonian Institution more than a century ago.

Another example is Helicobacter pylori, the bacteria that cause most stomach ulcers. When I was in medical school, the idea that ulcers were caused by bacterial infection seemed completely implausible. Now it's an accepted fact, with a number of diagnostic tools available to test for H. pylori infection. The infection hasn't changed, but our understanding and awareness of it has.

Global travel

People are traveling more often and to more-remote places than ever before — and so are germs. Because global travelers may be exposed to microbes to which they have no natural immunity, they're at increased risk of contracting an infectious disease, carrying it home with them and then spreading it to others. Between 2002 and 2003, infected travelers carried SARS from mainland China to Hong Kong, Singapore, Vietnam and Canada.

Globe-trotting animals and insects also can spread illness. West Nile virus probably made its way to New York City when an infected mosquito hitched a ride on an airplane from the Middle East. West Nile surfaced in Africa in the 1930s, and if it weren't for air travel, would likely have remained there. Instead, the disease is now found in Asia, Europe, the Middle East and most of North and South America.

Crowded conditions

In most parts of the world — and in developing nations in particular — cities and rural areas are becoming more densely populated and more interconnected. This makes it easier for germs to spread from person to person as well as from animals to people.

Many of the emerging infections we've seen recently — hantavirus, monkeypox, SARS and bird flu — have resulted from animal-to-human transmission. That's because viruses that cross the species barrier often originate in areas where people live in close proximity to animals, especially pigs, which are an ideal "mixing bowl" for virus genes.

At one time, people who lived cheek by jowl with animals — in rural areas in China, for instance — might have spent their entire lives on a single plot of land. But now, they're just as likely to travel to nearby towns and cities, sometimes inadvertently carrying new viruses with them.

Environmental change

Climate change, the destruction of habitat and the spread of humans into previously unpopulated areas have all contributed to the emergence of certain pathogens. When humans alter or encroach on a native habitat, especially one that is still relatively biodiverse, the stage is set for an emerging disease — not because a new microbe suddenly develops, but because humans come in contact with it for the first time.

Hantavirus is a case in point. It's likely that mice in the Four Corners region of the United States — an area shared by Colorado, New Mexico, Arizona and Utah where the syndrome first appeared — have carried the virus for hundreds or even thousands of years. But it wasn't until the early 1990s that shrinking habitat brought rodents and humans together, allowing the virus to spread to people.

Media attention

Emerging infectious diseases are very real. But we live in a media-saturated age with instantaneous reporting from all parts of the world. The result, for both good and ill, is that we hear about these illnesses to a much greater extent than ever before. And because sensationalized stories sell newspapers and magazines — and made-for-TV disaster movies — our perception of how common or threatening new diseases are is often skewed. Yet while the media exaggerate some threats, they ignore the fact that in many parts of the world, ancient scourges such as polio, smallpox, typhoid, cholera and scarlet fever have been successfully controlled.

How concerned should we be about emerging infectious diseases?

For people in developed nations, the risk of catching one of these diseases is very low. You're more likely to get food poisoning in your favorite restaurant than you are to contract an exotic illness. Yet caution is warranted, especially if you travel to an area where a particular infection is common. The best approach is to stay informed and take reasonable precautions, including — and this is both so simple and so critical — washing your hands thoroughly and often.

James M. Steckelberg, M.D.
Chair, Infectious Diseases
Mayo Clinic, Rochester, Minn.