Existence of Bacteria and Viruses in Hindu Puranas

The existence of bacteria and viruses are explained in the following text from the Mahabharata Shanti Parva Section XV

I also feel it advocates the Darwin's theory "Survival of the fittest"Arjun said: "I do not behold the creature in this world that supports life without doing any act of injury to others. Animals live upon animals, the stronger upon the weaker. The mongoose devours mice; the cat devours the mongoose; the dog devours the cat; the dog is again devoured by the spotted leopard. Behold all things again are devoured by the Destroyer when he comes.This mobile and immobile universe is food for living creatures. This has been ordained by the gods. The very ascetics cannot support their lives without killing creatures. In water, on earth, and fruits, there are innumerable creatures. It is not true that one does not slaughter them. What higher duty is there than supporting one's life? There are many creatures that are so minute that their existence can only be inferred. With the falling of the the eyelids alone, they are destroyed"