My daughter is 4 with delays and has had many bacterial and viral illnesses (aspiration pneumonia), but never gotten a fever. what might cause this?

Many issues: In deference to urban myth, fever does not cause developmental delays. Fever is a normal body response & those with good immune systems use it as needed.Aspiration pneumonia could have caused periods of decreased oxygen levels that starved the brain & some viral infections target brain cells causing injury.In about 1/2 the cases we never discover the cause after thorough evaluations.

Immunity: Have your daughter thoroughly evaluated by a pediatrician familiar with all immune deficiency disorders. Good luck.