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UNDATED (CNN) -- More than a dozen states are experiencing an increase in whooping cough cases. In Texas, it's now becoming an epidemic. So what's behind the spike?

Whooping cough is a brutal cough that can last for months and a potentially fatal illness, especially for babies and young children who haven't had a chance to get all their shots yet. Dr. Jason Bowling with the University of Texas Health Science Center says, "They actually may have spells of apnea where they stop breathing, so one to two out of 100 infants can actually die from this."

Sixteen states and Washington D.C. are seeing an increase in cases, compared to this time last year. More than 2,000 cases so far in Texas alone, claiming the lives of two babies.

The state is on course to have its worst whooping cough outbreak in over 50 years. Officials this week issuing a health alert, warning Texans that vaccinations are vital.

But the effectiveness of the whooping cough vaccine fades, quickly. by age 11, only 7 out of 10 kids are fully protected.

That's why the Centers for Disease Control recommend a booster shot for pre-teens to protect themselves. And any babies they could unwittingly infect.