The Na'vi are cells in a Gaian organism. Imagine if a culture of sapient bacteria came to Earth and tried to set up relations with our mitochondria. Their ecosystem should be at least a billion years older than ours - one in which symbiotic mutualists are the "next wave" after mammals. I would imagine that there are no diseases or parasites on Pandora - except for humans, and the movie itself showed us the immune reaction of a planetary sapience.
I'm reminded of both
A Case of Conscience and
Pennterra, both very fringe novels of ecological SF.