Something I never understood is why trees seem to be ill so rarely. There is a gazillion infectious agents on Earth from viruses to fungi, mammals are barely keeping up even with our adaptive immune system and fever. Plants lack all of that and yet some trees live for centuries or millennia when undisturbed.

I realize that are many factors skewing the observation: trees may be sick without us noticing, most of the trees die very early and we observe ones that get mature, some trees are in fact visibly sick etc. What I don't know is if these factors combined explain it all or not. My gut feeling tells me that without an adaptive immune response trees must be sick way more often than we see, but I have no proof.