HOUSTON -- Nearly 160 measles cases have been reported in Texas, the second largest US state, state health officials said on Monday, warning the outbreak could last for months.

A dozen measles cases were diagnosed in Texas over the weekend, bringing the state's total to 158, according to the Texas Department of State Health Services.

The cases stretch across nine counties, said Dr. Jennifer Shuford, the commissioner of the state health department, during a hearing with the public health committee in Austin on Monday.

"Due to the number of suspect cases that we have right now, we do expect more confirmed cases to occur in this outbreak," Shuford said.

"We know other outbreaks that have happened in the last 10 years across the United States have lasted months. Some as short as two and a half months, but the one in New York lasted around 10 and a half months," she added.

The first two cases were identified in late January and the numbers have been rising since, according to an NBC News report.

All cases are in unvaccinated people or those who have unknown vaccination status, said the report. Most cases are in children aged 5 to 17 years old.

Texas reported the first US death from the ongoing measles outbreak on Feb 26. The case was a school-aged child who was not vaccinated.

Measles is a highly contagious respiratory illness, which can cause life-threatening illness to anyone who is not protected against the virus.

Measles can be transmitted by direct contact with infectious droplets or by airborne spread when an infected person breathes, coughs, or sneezes. People who are infected will begin to have symptoms within a week or two after being exposed.

The best way to prevent getting sick is to be immunized with two doses of a measles-containing vaccine, which is primarily administered as the combination measles-mumps-rubella or MMR vaccine, according to the Texas Department of State Health Services.