Usually it starts with a heavy cold. Moisture drips from the nostrils, the bird gasps for air, and the breathing sounds squeaky and hissy. The bird is fatigued, often puffes out its feathers, and has diarreah. The final stages of the disease include symptoms of cramps and lameness. The picture below is of a bird infected with psittacosis.
Psittacosis can occur in a mild form, which can be completely cured.
Unfortunatly, be aware that birds can still be infectious for both birds and humans. At any rate, the disease can pose a hazard, which is why you should report any strange symptoms your bird displays to your avian vet. In humans, the disease starts with a cold and can lead to a lung infection. The disease can be treated with antibiotics, so long as you see your doctor in a timely manner. Make sure you tell your doctor you have a bird so they may properly diagnose you.