The number of measles cases associated with an outbreak in western Texas has grown to 259, according to new data released Friday.

Philadelphia hospital patients were also warned about possible exposure to the highly contagious virus just days after a warning for Dulles airport travelers.

Measles is caused by a highly contagious virus that's airborne and spreads easily when an infected person breathes, sneezes or coughs. It is preventable through vaccines, and has been considered eliminated from the U.

“In general, we have not recommended boosters because there is no clear evidence that people need it. But there probably is very little downside to the booster, so I know people who have gotten it. There is no evidence that titer levels necessarily correlate with whether you are protected or not.”