New Meeting Places
Any changes that create new intersections between microbes and people pave the way
for disease-causing agents to enter our species. One such change that has put us at risk is the global human population explosion—from about 1.6 billion people in 1900 to nearly 7 billion today. Humans have cleared forests for agriculture and suburbanization, leading to closer contact with environments that may harbor novel (or newly introduced) pathogens
. Through much of the world’s developing tropical regions, the massive expansion of roads and human settlements has also created transition zones
filled with opportunities for contact with potential disease-causing agents.
International trade and travel are associated with the emergence of such infectious agents as the SARS coronavirus and West Nile virus.
Human travel and commerce have brought other risks. Almost 2 million passengers, each a potential carrier of infection, travel daily by aircraft to international destinations. International commerce, especially in foodstuffs, adds to the global traffic of disease-causing microbes. Because the transit times of people and goods are often shorter than the incubation periods of infection, carriers of disease can arrive at their destination before the infection they harbor is detectable. International trade and travel are associated with the emergence of such infectious agents as the SARS
coronavirus and West Nile virus
Changes in human demographics and behavior are linked with the emergence of infections such as AIDS
and hepatitis C
, through sexual activity and intravenous
drug use. More broad-scale changes that raise the risk of infectious disease include the breakdown of public health systems, poverty, war, and famine.