Story highlights Montana has one confirmed case of Enterovirus D68

Four children in Lake County, Indiana, were sickened by Enterovirus D68

Enterovirus D68 worsens breathing problems for children with asthma

Indiana and Montana have joined the growing list of states with confirmed cases of Enterovirus D68 , health officials say.

Four children in Lake County were sickened by Enterovirus D68 and were treated at University of Chicago Hospital, according to a press release from the Indiana Department of Health; all four have since been discharged from the hospital. Lake County is in the northwest corner of the state.

Montana has one confirmed case of Enterovirus D68, says Jon Ebelt, a spokesman for the state's public health department. "A specimen sent to the CDC from a child under the age of 10 has tested positive for the particular strain of enterovirus," he told CNN.

Enteroviruses are very common, especially in the fall and winter months. The Centers for Disease Control and Prevention estimates 10 to 15 million infections occur in the United States each year. But this type of enterovirus -- Enterovirus D68 -- appears to be exacerbating breathing problems in children with asthma.

These viruses usually present like the common cold; symptoms include sneezing, a runny nose and a cough. Most people recover without any treatment. But if your child appears to be having trouble breathing, take him or her to a doctor right away.

As of September 11, the CDC had confirmed more than 80 cases of Enterovirus D68 in six states: Colorado, Iowa, Illinois, Kansas, Kentucky and Missouri. New York also announced on Friday that it had more than a dozen confirmed cases of the virus.

The Indiana State Department of Health is keeping an eye out for additional cases of enterovirus. Hospitals are sending samples from patients with severe respiratory illnesses to a state laboratory for testing.

Last week , media reports of kids flooding emergency rooms in Alabama and Washington state spoke to the spread of the virus. Other states, including Michigan, Georgia, Ohio, Oklahoma, and Utah, are also investigating clusters of respiratory illnesses. Several have sent samples to the CDC for testing.