(CNN)Despite outbreaks of flu, tuberculosis and chicken pox, US Customs and Border Protection refuses to tell the public how many migrants in detention facilities have contagious diseases.

Doctors have long been concerned that these congested settings are breeding grounds for serious illnesses.
"They create facilities that encourage the spread of infectious agents," said Dr. William Schaffner, an adviser to the US Centers for Disease Control and Prevention on vaccine issues.
A CBP senior health official told CNN his agency keeps track of infectious diseases among the hundreds of thousands of migrants who pass through CBP detention facilities but declined to release that data.