The number of measles cases is increasing. Here’s what you need to know about the symptoms.

Health officials are raising the alarm about the spread of measles in the United States and around the world.

On Monday, the Centers for Disease Control and Prevention issued a health advisory urging people over six months of age who travel internationally to get vaccinated against the highly contagious virus. The agency also said anyone returning to the United States from abroad should watch for measles symptoms for three weeks after returning home.

Here’s what you need to know about how the virus spreads and what its symptoms look like.

How and when measles spreads

Part of what makes measles so difficult to contain is that infected people can easily spread the virus before they even realize they have it. According to the CDC, someone with measles can transmit the infection for up to four days before developing a telltale rash

Measles spreads when someone who is sick coughs or sneezes, said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia. The virus can linger in the air for up to two hours after an infected person leaves a room. People can also get measles by touching a surface that a sick person has contaminated and then touching their eyes, nose, or mouth. “It is the most contagious, vaccine-preventable disease,” said Dr. Offit.

Symptoms to look out for

It usually takes one to two weeks for someone to feel sick after coming into contact with the virus. The first symptoms are usually coughing, runny nose, pink eye and fever. Some people may have a fever that rises above 104 degrees, which can be especially dangerous for young children. According to the agency, fewer than one in 10 people with measles will also develop diarrhea.

“It’s a pretty miserable disease,” said Dr. Sean O’Leary, a professor of pediatric infectious diseases at the University of Colorado School of Medicine.

Two to three days after symptoms first appear, people with measles may develop small white spots in the mouth, also called measles Koplik spots. A rash develops over the skin, usually about a day or two later, and often starts as flat red spots on the face that then spread to the neck, torso, arms, legs and feet. The rash usually goes away after about six days, according to the World Health Organisation. The CDC recommends that people isolate for four days after developing a rash because they can remain contagious during that period.

Most cases of measles are mild, but the virus can lead to serious complications, especially in infants, very young children, and people with compromised immune systems. The disease can be fatal. Pregnant women who are not vaccinated and contract measles may give birth prematurely or have a baby with a low birth weight. One in ten children with measles will develop the disease ear infection. Measles can also lead to pneumonia, the most common cause of death from measles in young children, and can cause blindness. Children can also develop brain swelling that can lead to deafness or intellectual disabilities.

Vaccination is the best way to protect against the disease, said Dr. Offit. Doctors generally recommend that children be vaccinated starting at age 1, although in some cases they can be vaccinated earlier. The CDC advisory encouraged parents traveling internationally to vaccinate infants older than six months.

Some people with compromised immune systems, such as those receiving chemotherapy, may not be able to get vaccinated. But for those who can get vaccinated, protection can typically last a lifetime, he said — and it’s never too late to get the shots.