An outbreak of measles in southwestern Washington is growing worse by the day, with the number of cases soaring to 50 as of Monday, according to state health officials.

Clark County health officials have confirmed 49 cases there since Jan. 1. On Jan. 23, one case was confirmed in King County, home to the state's largest city, Seattle. It is not clear where the King County patient became infected, but he did report having traveled to Vancouver, the seat of Clark County, before his diagnosis, health officials have said.

As with a similar outbreak among Orthodox Jews in New York state, nearly all of the infected were unvaccinated. Clark County reported that 42 of the 49 patients had not been vaccinated against measles. One patient had received only one of the two recommended doses of the vaccine, and the vaccination status of six others was unknown, the county department of health said.

People may have been exposed to the dangerous disease at more than three dozen locations, many in neighboring Oregon, such as Portland International Airport and the Moda Center, home to the Portland Trail Blazers. Other possible infection sites in Portland include an Amazon Locker location and stores such as Costco and Ikea.

On Jan. 22, Clark County health officials declared a public health emergency.

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Dr. Scott Lindquist, a Washington state epidemiologist, said the state health department is seeing new cases every day. "This is entirely preventable," he said in a statement to NBC News.

He advised families to check their immunization records. "If you are concerned your child may be developing measles, call your provider before going to a medical facility in an effort to prevent the spread of measles to vulnerable people within these facilities," he added.

A sign at the Vancouver Clinic in Washington warns visitors of a measles outbreak in Clark County. Gillian Flaccus / AP

Anti-vaccination hot spots have been developing for several years. Along with Washington and New York, at least seven other states — Hawaii, Oregon, Pennsylvania, Connecticut, Colorado, California and Georgia — have reported measles cases this year.

Reports that some people are self-medicating with vitamin A have also surfaced, leading the Washington State Department of Health to warn in a tweet that vitamin A has no effect on measles and that “the only way to avoid getting measles is to be vaccinated against it.”

Vitamin A cannot prevent or cure the measles. For a child with a healthy diet in the US, taking more vitamin A will not have any effect on their measles disease as they already get enough of it. The only way to avoid getting measles is to be vaccinated against it. pic.twitter.com/tYUNKNkGUJ — WA Dept. of Health (@WADeptHealth) January 31, 2019

Children who are deficient in vitamin A are at higher risk for severe complications from measles if they get the disease, but most children in the U.S. receive sufficient amounts in their diet. In the rare case of deficiency, vitamin A can be used to prevent severe complications of measles, including blindness and death.

Vitamin A is found in many foods, such as spinach, dairy products and liver. It is used as an oral supplement in people with poor or severely limited diets or in those who have a condition that increases the need for vitamin A, such as pancreatic or eye disease. It is also used in topical creams to reduce fine wrinkles, splotches and roughness and to treat acne.

The recommended daily amount of vitamin A is 900 micrograms for men and 700 for women. Taking too much at one time — over 200,000 micrograms — can cause nausea, vomiting, vertigo or blurry vision. More than 10,000 micrograms per day can cause long-term damage, such as bone thinning and liver damage. Pregnant women are especially cautioned against taking vitamin A, as it has been shown to cause birth defects.

The ongoing outbreak and the perceived need for home remedies is a reminder that herd immunity — the indirect protection of non-vaccinated people that occurs when a large percentage of the population has been vaccinated — has broken down.

The measles virus is very infectious. Before the vaccine’s introduction in 1963, there were four million measles cases in the U.S. every year, with 48,000 hospitalizations and 500 deaths.

The measles-mumps-rubella vaccine has been part of routine childhood shots for decades, and measles was declared eliminated in the U.S. in 2000. Those who get the proper doses of the vaccine rarely contract the disease even if they are exposed.

Children typically receive the first vaccine at 1 year to 15 months old, and the second dose at 4 to 6 years. If everyone receives the vaccine with proper doses, herd immunity is achieved.

Symptoms of measles generally begin a week to 14 days after infection. They include a high fever, cough and the signature bluish-white rash on the inner lining of the cheek called Koplik spots. If left untreated, encephalitis, a potentially fatal inflammation of the brain, can develop.