A 20-year-old student from University of California, Berkeley, might have spread measles to thousands of San Francisco Bay Area residents when he travelled through BART public transit system and attended classes while sick. The health officials said that the unidentified Contra Costa resident was not vaccinated and was more likely to have been infected with measles during a recent trip to Asia.

"Measles is a very serious viral illness and very contagious," said Erika Jenssen, communicable disease program chief for Contra Costa County Public Health, CBS Sacramento reports. "If this individual had been vaccinated and was exposed to measles abroad, they wouldn't have been infected and would not have brought the disease to the United States. And all of this would not be happening."

Kim LaPean, a university health services spokeswoman, said that UC Berkeley health officials have purchased 300 doses of measles vaccine for unvaccinated college students. "We'll continue to monitor the demand for the vaccination and may hold a clinic if necessary," LaPean said.

Disease experts said that vaccines are efficient and long-lasting. About 95 percent of people who receive one dosage of vaccine will be protected. Plus, people who already suffered from measles will not get affected again, SF Gate reports.

Despite its disappearance in 2000, measles cases have surfaced in recent years. According to Centers for Disease Control and prevention, about 189 people suffered from measles 2013, second largest figures in the country since 2000.

"A measles outbreak anywhere is a risk everywhere," CDC director Dr. Tom Frieden said. "The steady arrival of measles in the United States is a constant reminder that deadly diseases are testing our health security every day."

Measles spreads when an infected person coughs or sneezes in the air. After one to three weeks of contact, symptoms begin to appear including rash, high fever, runny nose, coughing and watery red eyes and sore throat, aches among others.

Around 30 percent of measles give rise to complications including ear infections, pneumonia or encephalitis.

"Measles can progress to death, but that is very rare," said Janet Berreman, health officer for the city of Berkeley said.