Measles outbreak continues in Texas, cases double: investigation underway

However, additional cases were detected in Lynn, Terry and Yoakum counties.

In every case, the patient was unvaccinated or their vaccination status was unknown, according to the DSHS.

The outbreak is under investigation.

The rash of cases comes more than two decades after health agencies reported the disease was "eliminated" in 2000.

Measles is a highly contagious respiratory illness that can be life-threatening, according to the DSHS.

It can be spread through direct contact with infectious droplets, or airborne when an infected person breathes, coughs or sneezes, according to officials. The virus can remain infectious in the air for up to two hours after an infected person leaves an area. 

Symptoms usually appear within a week or two, and include a high fever, cough, runny nose, and red, watery eyes. 

A few days later, flat, red spots appear on the face and then spread down the neck to the rest of the body. People are most contagious the four days before and four days after the rash appears.

Those who believe they may have measles, or may have been exposed to measles, should isolate and call their doctor before arriving to be tested, according to the DSHS. 

Fox News Digital's Christina Shaw contributed to this report.