For years, doctors noticed something strange about vaccines in India. Vaccines that worked well in Europe and America worked less well in Indian children. Same vaccine, same dose, weaker protection. Most people accepted it. One scientist in Vellore spent 30 years asking why. Her name is Gagandeep Kang. She trained as a doctor at Christian Medical College in Vellore, in Tamil Nadu, and then did something unusual for a physician. She went into the laboratory and stayed there. Her subject was the gut. Specifically, the guts of children in poor households. She studied what repeated diarrhoeal infection does to a child's intestine over the first years of life. What she found matters enormously. Those repeated infections do not just make a child ill. They damage the lining of the gut, stunt growth, and interfere with how the body responds to oral vaccines. That is why a vaccine developed in a clean environment abroad underperforms in a child who has had 10 infections before the age of 2. The problem was never the vaccine. It was what the child's gut had already been through. Then she went further than diagnosis. Rotavirus is the leading cause of severe diarrhoea in small children and it kills a very large number of them. India had access to imported rotavirus vaccines, but they were expensive and often out of reach for the families who needed them most. Kang led the clinical trials for Rotavac, India's first indigenous rotavirus vaccine, made by an Indian company. She was also central to the work behind a second one, Rotasiil. Both are now approved by the World Health Organisation. Rotavac went into India's national immunisation programme, which means it reaches children whose parents pay nothing for it. In 2019 she became the first Indian woman scientist working in India to be elected a Fellow of the Royal Society, the scientific body founded in London in 1660. It took 359 years. She has served on the World Health Organisation's global advisory committee on vaccine safety, and she now leads global health work at one of the largest health foundations in the world. She built her career on a question most people did not think was worth asking. Not how do we get vaccines to poor children. But why do they not work as well when they get there.