Now Playing: Neck collars providing answers to subconcussive impacts

Now Playing: What is ricin?

Now Playing: What are Typhus Fevers?

Now Playing: New advice on kids' concussions calls for better tracking

Now Playing: Salmonella outbreak linked to popular cereal

Now Playing: Globally 25 percent of adults aren't getting the recommended amount of exercise: WHO

Now Playing: Behind the byline: Barbara Feder Ostrov on her Prudence Hall story

Now Playing: FDA calls e-cigarettes 'an epidemic' among minors

Now Playing: Amazing advancements against cancer, but not everyone benefits

Now Playing: Overdose epidemic may have deeper social issues

Now Playing: Puppies to blame for sickening hundreds of people: CDC

Now Playing: Too much screen time, too little sleep linked to child development problems: Study

Now Playing: Newer hormonal birth control linked to lower ovarian cancer risk: Study

Now Playing: This day in history: Sept. 28, 2000

Now Playing: What are probiotics?

Now Playing: Genetically modified mosquitoes could eradicate malaria

Now Playing: Living in the city may not be heart healthy: Study

Now Playing: Secondhand smoke more dangerous for teens: Study

Now Playing: Could soccer 'heading' lead to greater brain injury for women than men?

Now Playing: Alcohol-based hand sanitizers: can they backfire?