Now Playing: 'Supergirl' toddler marks end of chemo by ringing bell during special send-off from hospital

Now Playing: New study in mice explores how breast cancer surgery may trigger cancer spread

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

Now Playing: Nearly 70 percent of early breast cancer patients may not need chemotherapy: Study

Now Playing: Teen's intense workout may have led to potentially fatal condition

Now Playing: What is the deadly Nipah virus?

Now Playing: Serena Williams speaks out about post-partum depression battle

Now Playing: Can working out in the cold help you burn more calories?

Now Playing: Baseball star describes recovery from brain aneurysm

Now Playing: Can working out too much kill you?

Now Playing: 'GMA' Hot List: What to know about the new colon cancer guidelines

Now Playing: What to know about new colon cancer guidelines

Now Playing: What new moms should know about a postpartum diet

Now Playing: New guidelines for colonoscopy screenings lower age to 45 from 50

Now Playing: How 3 women lost 100 pounds each without surgery

Now Playing: Some vitamins, minerals may lower risk of heart disease, stroke: Study

Now Playing: Recent study examines link between alcohol-related laws and drunk driving deaths

Now Playing: Meghan Markle sheds light on menstrual hygiene

Now Playing: FDA warns: Don't let your children sink their teeth into benzocaine

Now Playing: Dramatic rise in ADHD medication mishaps among kids and teens, report finds