Each autumn in the Sahel, a vast band of grasslands just south of the Sahara desert, seasonal farmers and their families move from their farms when the long dry season begins. Many travel long distances to large towns and cities where they squeeze into already crowded districts, finding spaces in extended family compounds or temporary sites on the city’s edges.
In places like Niamey, capital of the West African nation of Niger, the dry season also brings measles. Every autumn, a fresh outbreak. When the rains come in spring and the people return to their farms, measles cases drop off abruptly.
Was the virus itself affected by weather? Or, as researchers suspected, were the outbreaks related to the influx of seasonal migrants? Measles, after all, is highly infectious; it flourishes under crowded conditions. But with no good way to track the changing population in a densely populated place like Niamey, they had little chance to test their hypothesis.
Nita Bharti, now an assistant professor of biology at Penn State, began working on this problem as a postdoc at Princeton. “We knew these places had very important and predictable fluxes in population,” she says, “but no one had ever found a way to measure those changes.”
Bharti first looked at infectious disease as an anthropologist, with a focus on the role of behavior on epidemiology. When she began her Ph.D. research in biology at Penn State, she found that human behavior, especially long-distance movements, had been somewhat neglected as an important driver of infectious diseases.