Now Playing: Kellogg's recalls Honey Smacks after link to Salmonella outbreak

Now Playing: Foods with carbs and fats together make the brain light up more, study finds

Now Playing: What to do if you're stung by a jellyfish

Now Playing: Chemo may not be necessary for breast cancer patients: Study

Now Playing: Non-drinkers, heavy drinkers miss more work: Study

Now Playing: What is PTSD?

Now Playing: What is the deadly Nipah virus?

Now Playing: What is tick paralysis?

Now Playing: Apple Watch saves teenage girl's life

Now Playing: Why kids should stay away from sports drinks

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

Now Playing: Drug could hold cure for baldness

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

Now Playing: New recommendations for prostate cancer screening

Now Playing: EpiPen shortage anticipated by FDA

Now Playing: What is Ebola?

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

Now Playing: FDA approves preventative treatment for migraines