The tick-borne disease was first discovered in northwest Missouri in 2012 by a Heartland doctor.
Five of the new cases were reported in Missouri while the sixth case appeared in Tennessee.
Four of the six patients were hospitalized.
One person died but doctors are still investigating whether other factors played a role in his death.
All of the victims were between 50 and 60 years old.
The CDC said the virus is spread by the Lone Star tick which is commonly found in Missouri during the spring and early summer.
"The range of Lone Star ticks extends from northwest Missouri down south to Texas, southeast to Florida and all the way up to eastern seaboard to Maine," said Dr. Scott Folk. "We need to determine whether the Heartland virus is found within those geographic areas and then keep our eyes open for other human cases in other parts of the country as well."
Dr. Folk first discovered the Heartland virus in 2012.
Doctors recommend using an insect repellent with deet when going outdoors to protect yourself from tick bites.
Also, check for ticks as soon as possible after coming indoors and if you fall sick after a tick bite seek medical help.