organ system: have due flying around an higher blood circulation and metabolism level
But why do they have such immune system? Of course since millions of years they developed it by evolution. Flying around, eating different species of insects, spiders etc. and in case of fruit bats the fruits. And each insect and fruit has its own germs (viruses and bacteria and molds). Also fruits are manytime bitten by other species (birds, tropic squirrels, apes running on trees, etc.), leaving some fruit rests with infectious saliva, fecal and cells on the fruits.
Due to that continuous attacking by different germs of different species and due to the living in herd of millions and their ability to spread new germs cultures very fast they are predestined bio-reactors for new viruses and germs
Any (fruit) bat individuum which can not continue the fight / race between (fruit) bat organism and germs drops down to death (and therefore the caves of bat colonies are the real existing hell - darkness and missing solar UV-light, fecal many ft height, deadly germs, heat about 40°C, thousand insects like cockroaches eating bat fecal, poisonous snakes hunting bats, ticks, dead bats fallen down and skellets offering nutrition for such even dangerous insects, germs and perhaps also pangoline...)