I'd say it's because the city has nearly as many people living in it as the entire nation of Australia. Even if only 5% of them are sick, that's still a massive number and it would spread easily. Delhi is a bit of a no brainer, I'd say it was because of the intense dust there. Tamil Nadu, on the other hand, where my fathers family are from, and Sri Lanka, where my mum's side are from, rarely get me infectious diseases. I only get sick there if I drink water that hasn't been pre boiled.