Customs and Border Patrol will not provide flu vaccinations to migrants detained in facilities at the border, according to an official statement sent to CNBC this week. That decision came even though at least three children have died from the flu while being held by the agency, and despite an outbreak of the illness in May at a detention center in Texas.

"This goes against basic public health practice," says Paul Spiegel, director of the Center for Humanitarian Health at Johns Hopkins University. The Centers for Disease Control recommends everyone over the age of six months receive the flu shot, and in unsanitary and crowded conditions, like those reported at camps along the borders, the flu can pass from person to person more quickly than less congested areas.