Health officials say 118 cases of measles have been reported in the United States so far this year -- the highest number this early in the year since 1996. (The U.S. normally sees just 50 cases of measles in a year.)

The numbers in the UK are even more startling, with 330 measles cases reported in the first three months of this year -- nearly as many as all of last year, The Guardian reports:

The Centers for Disease Control and Prevention on Tuesday released the count for January 1 through May 20. Measles cases were seen in 23 states. None of the patients died, but about 40 percent were hospitalized.

What is measles?

Measles is a highly contagious respiratory infection, and up to 90 percent of people exposed to an infected person get sick. Symptoms include:

Fever and runny nose

Coughing and sneezing

Watery, red eyes

Rash of small flat spots

How is measles transmitted?

Similar to the common cold, sneezing and coughing release contaminated droplets that spread easily through the air. Unfortunately, the most contagious period of the disease occurs before the rash breaks out, so the risk of spreading the infection is very high, Leigh Vincour, M.D., spokesperson for the American College of Emergency Physicians, told The Huffington Post.

What can you do to protect yourself?

The widely accepted method of protection against measles is vaccination. About 90 percent of this year's U.S. measles cases were unvaccinated, the CDC reports. Health professionals encourage children to get the MMR (measles, mumps, and rubella) vaccine starting at an early age. According to the Mayo Clinic, "By 2000, the measles vaccine had practically eliminated measles in the United States. But there has been a recent resurgence of the disease, as more people have chosen not to vaccinate their children."

Why are people opting out of vaccination? From the National Institute of Health (NIH):

Some parents do not let their children get vaccinated because of unfounded fears that the MMR vaccine, which protects against measles, mumps, and rubella, can cause autism. Large studies of thousands of children have found no connection between this vaccine and autism. Not vaccinating children can lead to outbreaks of a measles, mumps, and rubella -- all of which are potentially serious diseases of childhood.

In April, Europe -- particularly France -- saw a major measles outbreak that health experts attribute to unvaccinated children. The World Health Organization reported nearly 5,000 cases between January and March.

Who is at risk?

According to the CDC, children too young to be vaccinated (generally, under a year old) and adults who skip the vaccine either by choice or due to medical conditions making immunization impossible, have the highest risk of developing the disease.

In addition to lack of immunization, risk factors include having a diet deficient in vitamin A, and international travel to and from developing countries where measles is more common.

An American traveler with measles caused a scare in February when her trip back from London potentially exposed travelers and workers in four major U.S. airports to the infection.