From water to air

Today I realised that most of the NICU(neonatal intensive care unit) everywhere are filled with babies with some problem related to their lungs. Which made me think, wow that’s a lot of problem and infection happening in one particular organ, which led me to think about which organ systems are exposed to the external world after birth and the answer was the skin, the respiratory system and the gastrointestinal system.
Which led me to think that bodies of terrestrial animals have to deal with infections from water AND air, unlike aquatic animals which have to deal with water born infections only (I’m not sure if that is factually correct or not, it just seemed correct in the moment).
Would that mean that animals had to have some major revamping of their immune systems before they could live successfully outside water?
Ooo that makes me wonder, were the flu viruses evolving as fast when there was no one to infect on land?
That makes me wonder, were there any airborne viruses when there were no living organisms on land?
Is there an aquatic equivalent on flu viruses?
And it goes on.
How do fishes protect their gills from infections? They seem to be more vulnerable than our lungs.
And how does mucosal immunity work for them? Isn’t water flowing over them continuously? Wouldn’t all the protein get washed away?
Or is there anything like a mucosal immunity in aquatic animals?
Well if you do know the answer to any of these questions let me know, I’d love to discuss and know more about what you think.