Several major Twin Cities hospitals and clinic systems announced precautionary limits on visitors Monday as four more cases of measles were reported by state health officials.

Minnesota's measles outbreak, which was first detected nearly four weeks ago, has sickened 48 people. Although most of the cases have been in children, a second infected adult is among the new cases announced Monday. That person is the parent of an infected child, according to the Minnesota Health Department.

Allina Health said Monday morning that it's asking that children under age 5 not visit its hospitals unless they are seeking care. All children under age 11 are asked to wear a mask while visiting Allina clinics or hospitals.

In addition, anyone with a cough or a sore throat is asked to wear a mask while in a hospital.

Allina joins Fairview Health System, which also has placed restrictions on children under 5 as well as anyone who is unvaccinated for the measles or who is ill. Anyone with possible measles symptoms is asked to wear a mask.

Allina owns Abbott Northwestern Hospital in Minneapolis, United in St. Paul, Mercy in Coon Rapids and nine smaller regional hospitals.

Fairview owns the University of Minnesota Medical Center, Fairview Ridges, Fairview Southdale and several smaller hospitals across the state.

Visitor restrictions by Minnesota hospitals are not uncommon during disease outbreaks. In January, Allina instituted restrictions because of the seasonal flu outbreak.

Among infectious diseases, measles is considered very contagious. About 90 percent of unvaccinated people who come in close contact with an infected person will catch the disease, according to the U.S. Centers for Disease Control. The measles virus can remain in the air of a room for up to two hours after an infected person has coughed or sneezed.

Measles is characterized by a rash that spreads to the entire body and is infectious four days before and after the rash develops. However, because the first symptoms of the disease often mimic those of a cold — fever, cough, watery eyes and runny nose — many patients don't suspect that measles is the cause.

This is the largest measles outbreak in the state since 1990, when 460 were sickened, including three deaths.

Twelve of the cases in the current outbreak have required hospitalization.

"We are very fortunate that there have been no deaths yet," said Kris Ehresmann, infectious disease director at the Health Department.

Most cases have been in Hennepin County, where 45 have contracted the disease. There are two cases in Ramsey County and one in Crow Wing County.

The outbreak has affected 41 Somali-Americans, in which vaccination rates are low, but last week the first cases developed in people outside that community.

"Our concern is that we are going to see further spread to the greater unvaccinated community in Minnesota," Ehresmann said.

Only three of those sickened patients had been vaccinated partly or fully against measles, including a health care worker who developed an infection after treating several patients.