Infectious diseases don’t need a passport to become global travelers. They hitch rides on airplanes, sail through customs unchecked and rapidly spread what might have started as an isolated outbreak to distant countries.

When that happens with viruses such as Ebola or Zika, the disease detectives and public health experts of the U.S. Centers for Disease Control and Prevention are often among the first responders a country calls for help.

Woman holding baby as health worker scans them for disease (© AP Images)
A mother and child are screened for Ebola in Sierra Leone in 2014. The U.S. Centers for Disease Control and Prevention trained 600 health workers across West Africa. (© AP Images)

Some of its experts may already be close at hand. “We have 1,700 people in more than 50 countries,” said Rebecca Martin, an epidemiologist who directs the Center for Global Health, the agency’s international arm.

While the agency’s mandate is to protect Americans’ health at home and abroad, it has long operated on the principle that, as Director Tom Frieden puts it, “it’s a lot safer, more effective and less expensive to stop threats where they emerge.” In doing so, it makes the world a healthier place for everybody.

During the 2014 fight to stop the Ebola epidemic in West Africa, the Center for Global Health sent in hundreds of staff members in waves. They went to do what Martin calls “bread-and-butter work” — helping countries build the capacity of their own public health systems to investigate and respond to outbreaks.

Field workers learn how to meticulously trace those who have come into contact with someone with an infectious disease. They knock on doors to find possible new cases, give diagnostic tests and enroll people in long-term studies. It was once routine to send specimens back to Atlanta for analysis, but now, with the center’s help, many developing countries operate sophisticated labs of their own, providing faster results.

Three women talking, one holding baby (© AP Images)
Global Health Center pediatrician Alexia Harrist (right) studies the Zika virus. She says goodbye after examining Janine Santos’ baby, Shayde Henrique, who was born with microcephaly in João Pessoa, Brazil. (© AP Images)

“We’ve worked for decades to ensure countries have the capacity to do this for themselves,” said Martin, who was posted to Africa for six years.

When an unfamiliar disease breaks out, the agency quickly publishes treatment guidance in its Morbidity and Mortality Weekly Report. “If you wait to get that into the traditional medical literature, [it will be] a year before clinicians see the information,” said Lauren Sauer, a Johns Hopkins University expert on emergency response.

The center does medical sleuthing across the United States, too, sometimes uncovering international connections, as when it traced a measles outbreak at Disneyland to a strain in the Philippines.

Worker in protective suit spraying insecticide in junkyard surrounded by tropical trees (© AP Images)
A municipal worker sprays insecticide in a junkyard in João Pessoa, Brazil, to combat mosquitos that transmit the Zika virus. A Center for Global Health team carried out a study in the town in Paraíba state on how to stop transmission of the virus. (© AP Images)

Dr. Daniel Bausch, a Geneva-based infectious disease specialist for the World Health Organization, said the U.S. health agency combines “a unique blend of epidemiologists and public health experts [with] a wealth of very, very good scientists.”

New threats such as Middle East respiratory syndrome (MERS), severe acute respiratory syndrome (SARS) and deadly flu strains have arisen fast and furiously of late, underscoring the importance of the agency’s international role, Bausch said.

“It’s like seeing a raging fire in the distance and saying, ‘Well, I’m not going to react until it gets to my house.’ Then it’s too late. The only real way to stop it is to go where the action is,” he said.