Michigan health officials have reported 22 cases of measles in metro Detroit since March 13, the state’s biggest outbreak of the disease in a quarter century.

The outbreak originated from an Oakland County visitor from Israel, according to health officials. Since then, 21 people in Oakland County and one in Wayne County have been infected.

Of the 22 patients, three had a record of being vaccinated, said Lynn Sutfin, spokeswoman for the Michigan Department of Health and Human Services.

About 1,400 people have gotten vaccinated in Oakland County since the outbreak started two weeks ago, Sutfin said.

Here’s what people need to know about measles:

1. Measles is highly contagious.

Measles is one of the most contagious diseases in the world. It’s so contagious that a measles patients can cough or sneeze, leave the room, and somebody else can be infected by walking in the room an hour or two later.

“Measles is so contagious that if one person has it, up to 90 percent of the people close to that person who are not immune will also become infected,” says the website for the federal Centers for Disease Control.

Before the measles vaccine was introduced in 1963, an estimated 3 to 4 million people got measles each year in the United States. Each year, about 400 to 500 people died from measles and 48,000 were hospitalized in the pre-vaccine era, the CDC says.

2. The acute stage of the disease lasts about five to six days.

Measles typically begins with a mild to moderate fever, often accompanied by a persistent cough, runny nose, inflamed eyes and sore throat. This stage last two to three days.

The next stage is a rash consisting of small red dots and bumps, starting with the face and spreading down the arms and trunk. The fever may spike as high as 105.8 degrees. The rash recedes gradually, fading first from the fast and last from the thighs and feet.

3. Measles can result in serious complications.

Measles can be serious in all age groups, especially adults and in children under 5, the CDC says. Among the complications, according to the CDC website:

Ear infections occur in about one out of every 10 children with measles and can result in permanent hearing loss.

As many as one out of every 20 children with measles gets pneumonia, the most common cause of death from measles in young children.

About one child out of every 1,000 who get measles will develop encephalitis, or swelling of the brain, that can lead to convulsions and can leave the child deaf or with intellectual disability.

For every 1,000 children who get measles, one or two will die from it.

Measles may cause pregnant woman to give birth prematurely, or have a low-birth-weight baby.

4, People with measles can spread the disease before they feel sick.

Measles symptoms generally appear about seven to 14 days after a person is infected, although it may take as long as 21 days. The person is contagious for about eight days, starting four days before the rash first appears and ending four days after the rash has broken out.

That’s been an issue in metro Detroit, where public health officers have a long list of places where measles patients might have contaminated others.

5. Measles vaccine is between 97 and 93 percent effective.

The MMR vaccine -- which covers measles, mumps and rubella -- is about 97 percent effective for who have had two doses and 93 percent effective for those who have had one dose.

Even if someone who is vaccinated gets measles, experts say those individuals will likely have a milder illlness and are less likely to spread the disease to others.

While the vaccine is highly effective, measles presents a danger to babies too young to be vaccinated, as well as to people who can’t be vaccinated because of allergies or compromised auto-immune systems.

6. If you’ve been exposed, the vaccine will work if given within 72 hours.

If you have been exposed to measles and you are not vaccinated, you likely can prevent the disease by getting a vaccine within 72 hours of exposure, Sutfin said. Also, gamma globulin shots are available for high-risk individuals who have been exposed and are not immune to the disease.

7. If you think you have measles, call before heading to your doctor’s office or an emergency room.

Because measles is so contagious, do NOT go to a doctor’s office or hospital without calling ahead first.

“Of course, you’re first inclination if you have measles is to see a doctor,” Sutfin said. “But obviously, coming into a doctor’s office, you could get a whole bunch of other people exposed. So we’re really stressing the importance of calling ahead and making arrangements.”