Andy, 18 months, had already seen 70 doctors and specialists, none of whom could tell his parents why he continued to have life-threatening, chronic infections.

Andy's arrival at Children's not only led to his infections being brought under control, it led to a breakthrough in doctors' understanding of the relationship between mutated genes and primary immune deficiency, which affects thousands of people in the United States.

Scientists, including Dr. Jordan Orange of Children's Hospital immunology department, had been studying what is known as a NEMO gene mutation, believing it to be a key to understanding and treating certain illnesses.

For instance, doctors knew that a consequence of a NEMO gene mutation was ectodermal dysplasia. But until Andy's arrival, they did not connect the NEMO gene mutation with primary immune deficiency.

A person with ectodermal dysplasia has poorly developed sweat glands, hair follicles, and conical shaped teeth caused by the NEMO gene mutation, said Dr. Hashish Jain, who studies immune deficiency at the Laboratory of Clinical Investigation of the National Institute of Health in Bethesda, Md.

Because Andy was so young, doctors did not know he had ectodermal dysplasia until his teeth came in, and were conically shaped.

Researchers then studied Andy's DNA in October 2000 and discovered that a mutated NEMO gene causes both primary immune deficiency and ectodermal dysplasia, Orange said.

NEMO is an acronym for the DNA sequence NF-kB Essential MOdulator. The NEMO gene essentially is a cell switch that sends instructions from the nucleus to different parts of a cell.

Orange will present the study's findings to the American Academy of Allergy, Asthma, and Immunology in March. He will focus on how to take care of children with ectodermal dysplasia and primary immunne deficiency syndrome.

Alone, either disease is treatable, but they can be deadly in combination.

Conference members will be told "how to make the diagnosis, when to make the diagnosis, and how to care for these children," Dr. Orange said.

Andres Trevino said his son now receives a gamma globulin treatment every two weeks, antibiotics, and antiviral medications for the primary immune deficiency. Gamma globulin is an antibody taken from donated blood and infused into the patient to strengthen the immune system.

Andres believes the best long-term treatment for Andy is bone marrow transplant or gene therapy.

Dr. Raif S. Geha, the chief of allergy, immunology, rheumatology and dermatology at Children's, said he believes a bone marrow transplant, in which stem cells are transferred to the patient to grow healthy blood cells is the most feasible long-term treatment for Andy. Gene therapy has been discontinued since two children in France developed leukemia from it.

Andres Trevino has not only traveled great distances, he has gone to great lengths to finance his son's care. He sells autographs of public figures, baseball caps, and phone cards inscribed with Andy's face.

The money is placed in "Andy's Fund," which he created in 2000, and is managed by the Children's Hospital Trust. Andy's Fund, $25,000 so far, supports research into immune deficiency by Dr. Geha and others.

John Sellars of Syracuse University is an expert on fund-raising and financial management.

He said though it is becoming common for non-profit groups, it is not so common for individuals to successfully raise money through the Internet, because donors may have a difficult time verifying sites' authenticity.

He said Andres Trevino's online fund-raising gains credibility because it is associated with Children's Hospital through the Andy Fund.

The Web site, www.andy.org.mx, describes Andy's syndromes and includes an explanation of the immune system, a list of Andy's hospital stays, names of his therapists and doctors, possible therapies and links to fundraising efforts.

Geha said Andy's treatment and associated expenses, about $300,000 since September 2000, have been covered by the hospital through National Institute of Health grants and research funding.

The family returned to Mexico City in early November 2000, but last March, for Andy's sake, moved to Auburndale to be close to Children's.

After 493 days of hospitalization, nine blood transfusions, 89 doctors, and numerous infections, Andy lives a normal life, Andres said.