A vegan diet does not support intensive (factory) farming of animals, which is a breeding ground for zoonotic disease. A large number of the infectious disease we suffer today originated with animals. Crowding animals together greatly increases the possibilities for disease to spread through the animals, and transition to the people who work with them.
Factory farming also makes use of large quantities of antibiotics to protect the animal housed in this un-natural way from disease. This leads to increased antibiotic resistance in animals and humans.
Factory farming, zoonotic disease and the risk of pandemics
It is pretty fair to say that if everyone was vegan there would be a lot fewer diseases for us to suffer and die from.