BT Slingsby was 13 years old when he made a career decision: to become a medical doctor.

Slingsby, now a doctor and CEO of the Tokyo-based Global Health Innovative Technology Fund, was living in Egypt with his family at the time. One day he saw a group of men smoking water pipes and hanging around on a street. One of them had huge, swollen legs below his knees — heavy like a brick and he could barely move.

The young Slingsby became curious and walked up to the man, who was probably in his 40s, and asked him about his legs. The man told him that he contracted a disease through a mosquito bite and that while he no longer had the disease, its complications left him with those gigantic limbs.