My parakeet sits on bottom of cage?

Is my parakeet sick? He is sitting on bottom of cage. When I put my hand in he squawks real loud. He never did this before. Also, if he is sick and his cage is next to my atiel...will my atiel get sick? Do I take my parakeet to the vet? What would they do? Should I wait and see if he gets better on his own? Many thanks.