Evolution of Humans “left defence less” Nature has left us defenceless we humans do not possess any natural way of defence self protection or methods of survival." But I have some good news for you: saliva has been the medicine that helps us humans to survive noxas. And not only humans but also lions, bears, wolves and all land mammals. You will be surprised when you know what a large number of diseases can cure this the most natural of all medicines.
When you say this: "With the innumerable diseases why is it hard to find that they keep attacking humans only why don’t we hear a dog getting malaria or a cat catching a cold or suffering from any allergy, why is it that only we humans are prone to attacks from the microscopic world ?" The answer is this: they do not have that large number of mental originated diseases we do have, which prepare the terrain for microbes to act in the body. And yes; they heal themselves with... saliva!!; they cure themseves and others with their saliva:colds, cough, inflammations, infections, skin conditions... You will see. Be ready for surprises, nice ones.
Just one more line: we used saliva a great deal until very little time ago: grandmothers still know about it and have a strong knowledge.