From January 2017 to April of this year, the CDC has received more than 2,500 reports of infections from California, Indiana, Kentucky, Michigan, Utah and West Virginia.

"Person-to-person transmission of [Hepatitis A] between persons who report drug use and/or homelessness could result from contaminated needles and other injection paraphernalia, specific sexual contact and practices, or from generally poor sanitary conditions," the CDC said in its report. 

Hepatitis A can be prevented with a vaccine, but economic instability, limited access to health care, and distrust of public officials make it difficult to reach the homeless and people using drugs, the CDC said. 

An outbreak of Hepatitis A of this magnitude is unprecedented in the U.S. 

In the mid-1980s, drug use was a risk factor for more than 20 percent of cases reported to the CDC, but no large outbreaks have occurred among people who use drugs since 1996. 

And while outbreaks of Hepatitis A among homeless people have occurred in other countries, it has not previously happened in the U.S. 

"Rapid identification, a comprehensive response, and novel public health approaches may be required to address needs unique to these populations. Urgent action is needed to prevent further HAV transmission among these risk groups," the CDC said. 

Source:   http://thehill.com/policy/healthcare/392271-cdc-investigating-hepatitis-a-outbreak-in-six-states