Amid one of the largest measles outbreaks in the United States in nearly a decade, anti-vaccine activists are promoting false claims that shots are ineffective in stemming the spread of the virus or that they could cause the disease.

As a measles outbreak sweeps through Texas, officially sickening 124 people, mostly unvaccinated children, and hospitalizing 18, anti-vaccine groups are pushing a familiar and false theory: The highly contagious virus is being caused by the vaccine itself.

Measles has sickened nearly 150 mostly unvaccinated people in rural West Texas, and a school-age child has died.

Infectious disease doctors said the measles virus can cause serious complications, and they say now is the time to make sure your children are vaccinated.