Now Playing: FDA approves preventative treatment for migraines

Now Playing: Young American smokers get half of their smoke through hookahs: Study

Now Playing: Apple Watch saves teenage girl's life

Now Playing: What is E. Coli?

Now Playing: New recommendations for prostate cancer screening

Now Playing: Why kids should stay away from sports drinks

Now Playing: Weight training can help shape the body and also the mind, studies show

Now Playing: Should we be concerned with second-hand pot smoke, especially around children?

Now Playing: Could a PG-15 rating be coming to a theater near you?

Now Playing: The lowdown on how to keep eggs in your diet while watching your cholesterol levels

Now Playing: EpiPen shortage anticipated by FDA

Now Playing: Drug could hold cure for baldness

Now Playing: What is Ebola?

Now Playing: Study shows rear-facing car seats perform well in rear-end crashes

Now Playing: Health officials warn of 'nightmare' bacteria

Now Playing: What is the definition of twins?

Now Playing: Family history of heart disease doesn't mean you shouldn't exercise, study shows

Now Playing: Hot off the press: Can peppers cause headaches?

Now Playing: Later school start times help teens' moods, study says

Now Playing: Moms whose babies have heart defects may have long-term heart health risks