In America

In 1982, Johnny and Sarah Sullivan and their daughters Christy and Ariel enter the United States on a tourist visa via Canada, where Johnny was working as an actor. The family settles in New York City, in a rundown Hell's Kitchen tenement occupied by drug addicts, transvestites, and a reclusive Nigerian artist/photographer named Mateo Kuamey. Hanging over the family is the death of their five-year-old son Frankie, who died from a brain tumor. The devout Roman Catholic Johnny questions God and has lost any ability to feel true emotions, which has affected his relationship with his family. Christy believes she has been granted three wishes by her dead brother, which she only uses at times of near-dire consequences for the family as they try to survive in New York.
After finding the apartment, Sarah gets a job in the local ice cream parlor to support the family while Johnny auditions for any role for which he is suited, with no success. Despite their poverty, the initial joy of being in the United States and the closeness of the family gives them the energy to make the most of what they have, and Christy chronicles the events of their life with a cherished camcorder. But as money runs low and the city's temperatures soar, tensions between Johnny and Sarah begin to rise with them. Not helping their financial and emotional strain is the discovery that Sarah is pregnant. Johnny finds work as a cab driver to augment their income and help pay for the girls' Catholic school tuition.
On Halloween, the girls become friendly with Mateo when they knock at his door to trick-or-treat. Despite Johnny's reticence about the somewhat imposing and forbidding man, Sarah invites him to their apartment for dinner, and eventually they learn that the man is sad and lonely because he is dying of AIDS.
Later, Mateo falls down a flight of stairs and is knocked unconscious. Christy tries to resuscitate him using CPR, although she is warned away from him by the other residents, who seem to be aware that he is HIV-positive. The man's condition continues to deteriorate as Sarah's fetus develops. The baby is born prematurely and in poor health, and is in need of a blood transfusion. Johnny and Sarah are ultimately nervous not only about the baby's survival chances, but also of the skyrocketing hospital bills that will now need to be paid following the baby's delivery, causing Sarah to have a brief nervous breakdown and blame Johnny for Frankie's death, and tearfully berates him.
However, after calming her down, Johnny and Sarah agree to the blood transfusion but without giving the baby "bad blood," as using hospital blood banks was the source of Mateo's contraction of HIV. Shortly, it is discovered that Christy has a compatible blood type to donate with, and Mateo's death coincides with the first healthy movements of the infant following a blood transfusion from Christy. After the successful operation, the family is startled to learn that Mateo had settled and paid for their astronomical hospital bill before he had died, upon the discovery that Mateo was in possession of a large trust fund he never spent. In return for his generous act, they give the newborn baby girl the middle name of Mateo in gratitude and to honour his memory.
With the birth of the new baby and the death of Mateo, Johnny finally is able to overcome his lack of emotion and put his grieving for Frankie to rest. He also finally catches a break by getting a small role in A Chorus Line on Broadway. The film ends after a baby shower at the apartment is held for the Sullivan family with many of the apartment tenants present to celebrate, and Christy and the rest of her family overlook the view of the city and look out for Mateo in the night's sky.

Question: Why did the residents warn Christy ?
Answer: The residents were aware that Mateo is HIV positive so they warn Christy of this.