Emerging infectious diseases present new threats to people living in the 21st century. Infectious diseases spread from one organism (person, animal, or plant) to another. Each emerging infectious disease meets one or more of the following conditions: The disease was previously unknown. It has appeared in a new place where no one expected it. It is familiar but now affects far more people than ever before. Once under control, the disease now may resist the antibiotics used to kill it. Or insects carrying the disease may have become resistant to the insecticides.

Medicine has made huge strides in the last few centuries. Smallpox, once a common killer, is almost eradicated. Vaccines save billions of people from measles, polio, and other terrible diseases. Better sanitation and nutrition have enabled humans to outwit cholera, scurvy, and other age-old problems.

Vaccination campaigns, such as this Nigerian poster, helped humankind defeat smallpox. In 1979 the world was declared smallpox-free. Source: Center for Disease Control and Prevention

NEW ERA OF EPIDEMICS?

Despite all our progress, though, we could face a new era of epidemics. That's largely because many diseases flourish in hot weather. Malaria, yellow fever, dengue fever, and schistosomiasis, for example, thrive in the tropics. A warmer Earth could make it easier for these ailments to spread, infecting more and more people. And diseases that currently peak in the summer, at least in temperate climates, could be transmitted all year long.

Get a quick overview of major diseases afflicting our world. Options: 56k | 220k (RealPlayer required) Source: Journey to Planet Earth

Some of these diseases, like malaria, spread because of international airplane flights that transport either infected people or mosquitoes. West Nile virus is spreading across the U.S., along the migratory pathways of birds. Right now, more birds than people suffer from this mosquito-borne disease, but it is expected that birds will soon transport the disease to every state.

Source: Center for Disease Control and Prevention

Climate change could help West Nile virus spread. Warmer winters would allow mosquitoes to survive longer. And severe summer drought may bring birds and
mosquitoes in closer proximity to scarce water holes.

THE MISERY OF MENINGITIS

The meningitis epidemics that afflict sub-Saharan Africa offer a striking example of the links between climate and disease. This painful and potentially fatal illness tears through communities during the dry season. It subsides once the rainy season starts. Why? Much more dust blows around during dry spells than during rainfall. And the dust provides easy transportation for the bacteria that causes meningitis.

Source: United Nations

Meningitis causes an inflammation of the membranes around the brain. Symptoms include a stiff neck, fever, and nausea. Meningitis also occurs in the U.S., and many college students are vaccinated to prevent epidemics.

MOLDS AND FUNGI

Climate change could also encourage the growth of molds and other fungi. They can cause skin rashes, respiratory problems (such as asthma or pneumonia), and other ailments. Many types of molds and fugi thrive in heat and humidity. Climate models show that some places will get more rain than ever before, giving molds a better opportunity to grow.

Even uglier than this ceiling are the health problems that could result from all that mold. Source: Environmental Protection Agency

Given all the diverse ways that climate can affect the spread of disease, scientists warn that we need to think seriously about emerging infectious diseases (See Science & You, below)—illnesses that are new or appear in new ways. Are our actions giving such illnesses a warm welcome?

What exactly is an "emerging infectious disease"?