WATCH: Men might soon use this gel for birth control

  • Now Playing: US life expectancy drops for 3rd year in a row


  • Now Playing: World AIDS Day: Through the years


  • Now Playing: Romaine lettuce from California linked to E. coli, FDA says


  • Now Playing: What is ‘bloating’?


  • Now Playing: Cigarette smoking in the US hits record low, but it’s not all good news


  • Now Playing: New study shows actual force backpacks put on your spine


  • Now Playing: Maryland college student dies of adenovirus


  • Now Playing: What is diabetes?


  • Now Playing: Top tips for Thanksgiving food safety


  • Now Playing: What is AFM?


  • Now Playing: CDC warns against eating romaine lettuce after E. Coli outbreak


  • Now Playing: Imagining your fears can help you conquer them


  • Now Playing: What is ricin?


  • Now Playing: What are Typhus Fevers?


  • Now Playing: Aerobic exercise can help relieve symptoms of depression


  • Now Playing: What is multiple sclerosis?


  • Now Playing: Rate of unvaccinated toddlers increasing


  • Now Playing: Medicine isn’t candy: How to keep your kids safe this Halloween


  • Now Playing: Feb. 7, 2001: Rod Stewart opens up to Diane Sawyer about thyroid cancer diagnosis


  • Now Playing: Neck collars providing answers to subconcussive impacts