Their 19-month old son Andy had been constantly sick since birth, and his parents did not know what caused his illnesses. He had just spent 23 days at Hospital ABC in Mexico City, the family's home at the time, and prior to that, he had spent over a month at Children's Hospital in Boston, undergoing treatment for various infections.

Andy had just returned home on Dec. 30, and the family planned a small New Year's Eve celebration. But that day, Andy woke up from a nap and accidentally removed the parenteral nutrition system that fed nutrients into his veins.

The Treviños were frantically searching Mexico City for a surgeon on New Year's Eve, so the baby could have a tenth control line put into his body, and Andres Treviño realized that enough was enough.

In April 2001, the family moved to the Boston area to seek the best possible treatment for Andy.

"It was obvious that when we were here, Andy was OK," said Treviño.

Andy, now a sweet, moon-faced four-year-old, suffers from Primary Immunodeficiency, which was caused by a genetic mutation on the Nuclear Factor Kappa B Essential Modifier, or NEMO, gene. His natural killer cells, which provide the immune system's defenses, do not work, and he currently takes a mixture of antibiotics, antiviral drugs, and steroids that keep him stable.

His best chance at long-term survival, according to doctors, is a bone-marrow transplant, and the Treviños are currently searching for a donor.

The Treviños have learned much about genetics, infections, and immune systems since their ordeal began when Andy was born in 1999, but they have also learned equally important information about doctors, hospitals, and taking care of sick children - and they were eager to share their knowledge with frightened parents.

Two years ago, Andres Treviño set up The Andy Fund, a Web site with a dual purpose: to raise money for both Andy's treatments and genetic research, and to help parents through their children's illnesses and extended hospital stays.

"We've learned about a lot; we've met a lot of parents and children in the hospitals, and we have many wonderful stories and frightening stories from being there," said Treviño. "If we could do something to help parents cope with spending time in the hospital, that will make us happy."

Andy was born on May 15, 1999, and for the first 48 hours of his life, nothing seemed wrong. Paulina's pregnancy was normal, said Andres, and the natural birth went as planned.

On May 17, Andy developed a fever, and he didn't want to eat. His parents brought him to Hospital Angeles in Mexico City, where doctors determined he had sepsis, a blood infection.

A pediatrician told them it wasn't normal to have an infection so soon after birth, but Andy was treated with antibiotics for 10 days, and went home.

A month later, he was back in the hospital with a stomach infection, and the Treviños' troubles began. Before his first birthday, Andy had spent 164 nights in the hospital; his condition was not diagnosed until he was two.

"I've never been in a hospital myself, for illness or anything," said Andres Treviño. "So dealing with all the doctors and procedures was difficult at that time."

Andy suffered from severe diarrhea, stomach infections, meningitis, and repeated bouts of cytomegalovirus. He was fed with a parenteral nutrition tube, then a nose tube until he developed pneumonia.

He had antibiotics brought to him from Los Angeles, spent time at the Children's Hospital in Denver, and underwent almost a barrage of immune-deficiency test.

His parents were frustrated with what seemed like a lack of progress, and tired of trying to balance all the specialists in Mexico City, all of whom worked in different hospitals and did not always want to collaborate.

In addition, they were far more impressed with the medical care that Andy received in the U.S. than in Mexico. The nurses seemed more knowledgeable, and the procedures easier.

"We saw that nurses there had more experience, and we felt more secure that they knew what they were doing," said Treviño.

Finally, a surgeon in Mexico City told them he did not think the medical system had the resources to treat Andy properly. The Treviño family thanked him, called a gastroenterologist at Children's Hospital in Boston, and told him they would be arriving the next day. The doctor protested, saying that travelling might not be safe for Andy.

"I said, 'We are desperate and we're gonna be there tomorrow,'" said Treviño.

On Sept. 9, 2000, the Treviños arrived at the Children's Hospital emergency room and checked Andy into the hospital. They began treatment with Dr. Jordan Orange, an immunologist who told them there was one test that nobody had given Andy yet: the Natural Killer Cell Function Test. Orange tested Andy's blood, and found that his killer cells weren't working.

"That was the first time we found something about his immune system," said Treviño.

A second major clue came during that same stay at Children's, when a dermatologist diagnosed Andy with the skin disease ectodermal dysplasia. Andres Treviño contacted the National Foundation for Ectodermal Dysplasia, which put him in touch with a scientist in Oregon, who ultimately provided the missing link in the cause of Andy's illness: a mutation on the fourth part of the NEMO gene.

"We were feeling that we were getting Andy's diagnosis," said Treviño. "Now we had to focus on what to do to cure him."

Andy is the only known patient with this particular mutation, according to Dr. Orange, although there is at least one other patient with a similar mutation. A team of doctors at Children's spent the next two years figuring out a combination of medications that would keep Andy relatively healthy and functional.

At age four, he attends school in Waltham, likes to play with cars and planes, and loves animals. But there is no way of knowing how long the medications will work for him, according to Orange, who estimated that Andy's stability "could range from minutes to years."

"It is an extraordinarily fragile balance that we have presently arrived at," wrote Orange in an e-mail.

The side effects of his medications range from a possibility that they will further suppress his immune system to a weakening of his bones from the steroids he takes. Just last week, while playing in the park, he broke his leg jumping off of a piece of playground equipment.

Treviño worked for a telecommunications firm in Mexico, but he declared bankruptcy on the 150th day of Andy's hospitalization, realizing that he simply could not pay for all of Andy's treatments. The family relies on donations, help from friends and family, and the rent from their Mexico City apartment to cover their expenses; taking care of Andy and maintaining the Andy Fund Web site is now Treviño's full-time job.

From their Newton apartment and often from the hospital, he updates their friends and family in Mexico on Andy's condition, and provides a resource for parents of sick children.

Over the months they have spent in hospitals, the Treviños have met many other families suffering through a similar ordeal. One father, he said, was a prisoner of war in Kuwait years ago, and Treviño asked him which was worse: being a POW, or being a hospital parent.

"He said, 'Well, when I was a prisoner of war, I was by myself, and I knew that someday I would be able to go out. In the hospital, it's your son or daughter that is in, and there is little that you can do.'

"It's very difficult to see a child suffering," Treviño added.

The site's guest book is filled with best wishes from friends and family, notes from Andy's nurses at various hospitals, and even the occasional note from a family that has learned something from the site.

Treviño's favorite message was posted just last month, from a family that was able to identify their son's immune disorder without going through the years of sickness and infection.

"We have benefited by all the knowledge learned from you and the others countlessly already," the message reads. "Stevie is here with us and doing so well largely because of you, and we want to thank you for that, from the bottom of our hearts."

The Treviños know they have a difficult time ahead of them as they try to maintain Andy's health and search for a compatible bone marrow donor. But a note from a family who has learned from Andy's experience makes the journey a little bit better.

"We didn't choose for Andy to be born like this," said Treviño. "But we try to make it as best as we can."