Unbalancing Act

Migration: Microbes on the Move

Every day, hundreds of millions of people take to the road, to escape war, seek a better life, or go on vacation. Uninvited guests—disease-causing microbes—often hitch a ride.

As of early
has infected
about 60 million people worldwide. It has increased by 50 percent since 2001.

Source: UNAIDS

The virus has probably existed in isolated jungles of central Africa for hundreds or thousands of years.
Scientists presume that people became infected by eating an infected primate or being bitten by an infected primate.
But neither infected people, nor the virus ever traveled far from these remote areas.
In the mid-20th century, however, travel became much easier, making the spread of HIV/AIDS possible. The result: our current pandemic.

One major worry around the world right now is a brand-new deadly disease called SARS—Severe Acute Respiratory Syndrome. A virus, which experts believe jumped from farm animals to humans in China, causes SARS/. The disease spreads easily and researchers are working hard to develop vaccines and treatments. For updates, visit the World Health Organization .


Wars frequently force refugees to live in conditions that promote the spread of disease. Source: UNHCRWars frequently force refugees to
live in conditions that promote the
spread of disease

Source: UNHCR


Throughout human history, people have unknowingly carried disease-causing microbes from place to place, often with disastrous results. In the centuries after Christopher Columbus first landed in the Americas, Europeans brought infectious diseases that killed tens of millions of people—80 to 90 percent of the native population.



click to enlarge imageIncreased international travel. Source: WHO
Source: World Health Organization


Globalization and growing international trade also contribute to disease. Some major roadways in India, for example, are known as "AIDS highways" because the long-distance truck drivers who use these highways carry the virus. Experts estimate that a half-million truck drivers in India are HIV/positive.

As international travel becomes more affordable and a whole lot easier, more than 600 million people worldwide take airplane trips each year. Those who travel by car, bus, ship, train—and by foot—are just too numerous too count.

Plane travel especially fuels the spread of disease. One of the strangest phenomena is called "airport malaria"—which strikes people who live near airports in countries that have no malaria (until an infected mosquito or person gets off the plane). In Paris, France, for example, neighbors of the international airport who have never traveled to malaria-risk countries have come down with falciparum malaria—the most serious form of the disease.

Click to see how quickly the mosquito-borne West Nile virus, which first appeared in the U.S. in 1999, is spreading.


Regional origins of immigrants to the U.S. Source: Population Reference Bureau
Most immigrants to the U.S. now come from Latin America and Asia
Source: Population Reference Bureau


People flow from poor economic areas to places that offer jobs and opportunity. Mexicans go to the United States, Turks, to Germany, and Indonesians, to Malaysia. Some even move from one part of their own country to another. More than 100 million workers in China, for example, have migrated to the economically prosperous provinces along China's coasts.

When migrant farmworkers clear rainforests and do other kinds of work that bring them into contact with mosquito-borne diseases such as yellow fever. These workers sometimes get more than they bargained for, bringing home both the diseases and the mosquitoes that carry the diseases. Learn more about yellow fever, which hits about 200,000 people each year—in cities and in rural areas.


Major  refugee populations, 2001. Source: UNHCR
All of the countries in this chart have recently experienced major wars
Source: UNHCR

Movement of people caused by war contributes most to disease. In Central Africa, for example, decades of almost continuous wars have produced at least 15 million refugees—people who must live year after year without homes or even the most basic sanitary facilities. It is no coincidence, then, that some of the world's worst and most threatening disease epidemics—including meningitis, cholera, and Ebola —start in Central Africa.


One of the reasons the situation is so serious is that microbes are such good travelers. Here is one experience that caused scientists to be super cautious when the astronauts returned from the moon in 1970:

  • A surprising thing happened in 1969. When astronauts from Apollo 12 landed on the moon—they found bacteria from Earth! How did that happen?
  • The bacteria sat inside a camera that had been on the moon since 1967, when an unmanned NASA spacecraft had landed.
Bacteria.  Source:  MicroAngela
Some bacteria can thrive in extreme environments-
from boiling hot springs to polar ice

Source: MicroAngela
  • Scientists had not sterilized the camera before sending it to the moon, because they were sure that microbes could not survive the journey.
  • But plenty of microbes had survived the vacuum of outer space, exposure to deadly space radiation, extremely low temperatures, and lack of food for more than three years!

This shows just how tough microbes can be, and how difficult it can be to keep them from tagging along. Click here to get the bigger picture about bacteria and other small parts of our world.

Get more fun facts about microbes and space from NASA.


science_icon How do officials keep epidemics from spreading?


Can we create healthy cities? back_unbalancenext_unbalance Can we put the brakes on disease outbreaks?

Additional information