The death of a 72-year-old woman with a pet cockatoo prompted medical researchers on Monday to warn people with compromised immune systems to avoid pet birds and their droppings. The woman was infected with a microscopic fungus called cryptococcus neoformans, or C. neoformans, a known danger to the health of immunosuppressed patients with diseases including AIDS. The same pathogen, researchers said, has been found in the feces of 26 percent of canaries, 18.4 percent of carrier pigeons and at lower rates among parakeets and parrots.