Cancer seized control of Andre Messier’s 7-year-old body so fast that it almost killed him before anybody knew it was there. In a matter of days it bred a large tumor in his stomach, and replaced virtually all of his bone marrow.

Then the doctors struck back, just as fast and hard, with drugs still more powerful than the cancer — and nearly as dangerous to Andre.

Now Andre sits near the window of his hospital room, watching the helicopters buzz back and forth over South Boston, while traffic and the tiny dots of pedestrians weave in and out on the street below.

Even while the boy from Bangor ponders the speed of city life, his family, friends and a battery of medical personnel try to keep tabs on the lightning-quick thrusts of cancer and drugs in Andre’s body. And they keep a wary eye out for the infections that would easily overwhelm what little defense Andre’s body has left.

Cancer, drugs, infection, Andre — in a few more days the winner will emerge.

Andre’s rare disease, Burkitt’s Lymphoma with bone marrow involvement, first appeared on Aug. 10, during what doctors had intended to be an appendectomy. Instead they found and removed a large tumor and a part of his intestine.

Just a day before that, Andre was outside playing in his yard in Bangor, a young boy enjoying summer, except for a persistent stomach ache.

“I remember I looked out the window and saw his bony body pushing that lawn mower,” says Lynda Messier, Andre’s mother. “I believe the word `lymphoma’ had been mentioned to me, but frankly I did not know what it meant, and the chances were so slim that I didn’t ask.”

Against the odds, it turned out to be lymphoma. Against still greater odds, it turned out to be Burkitt’s, a cancer most often found in Africa, where it has been linked to a virus. It also appears in a very few western children who have a genetic disposition to the disease. Burkitt’s is one of the fastest of cancers, doubling itself in as little as 24 hours.

Three days after the abdominal surgery, Andre was on a helicopter to Boston’s New England Medical Center, where Dr. Molly Schwenn immediately put him on an experimental program of treatment that both accelerates and intensifies the chemotherapy normally used to fight cancer.

Schwenn is an authority on Burkitt’s. Frustrated with traditional methods of fighting Burkitt’s, which saved “considerably less than 50 percent” of its victims, she developed an innovative treatment: four drugs, with dosages ranging from normal to 60 times the traditional amount, administered as quickly as the body could survive. Patients undergo four different blasts of chemotherapy in the first 22 days of treatment alone. The drugs kill the cancers cells, but they also kill body cells that are dividing, such as white blood cells.

“We’re not really allowing the patients’ tissues to recover between treatments. That’s why we run the risk of toxicity,” Schwenn says. “But the Burkitt’s cells have such fast turnover, you have to get ahead of the tumor.”

If a thorough scan on the 42nd day of treatment reveals the presence of Burkitt’s cells, the treatment is considered to have failed and is discontinued. If the Day 42 test reveals no cancer, treatment continues for another 24 days, to make certain every last malignant cell is killed.

In the last six years, Schwenn has used the radical treatment with children from around the nation and as far away as Greece and Israel. Of the 30 patients, 75 percent have lived two years without further event. While the term “home free” is not in a doctor’s vocabulary, Schwenn says that patients who live two years after the discovery of Burkitt’s are not likely to relapse.

Schwenn says that Andre is remarkable, both in the degree to which Burkitt’s had taken over his body, and in the speed of his recovery. A thorough test after 14 days of treatment showed only traces of the cancer remained. The next such test, on the 42nd day, will take place next week. His reaction to the drugs — nausea, weakness, pain in the mouth, bladder and other areas — has been controlled, and despite frequent fevers, no infection has yet been detected.

Schwenn is the first to admit that Andre has more in his favor than simple chemistry, however.

“The support of a family is extrememly important, and in their case, they have a strong faith as well, and that helps,” the doctor says.

Mrs. Messier has moved into her son’s hospital room. She sleeps on a special Barcalounger the hospital has provided to encourage parents to stay with their children.

Her presence, along with a few toys and the 219 get-well cards that line the walls, helps soften the isolation of the room, which Andre cannot leave. To minimize the possibility of infection, the room has its own air filtration, and a special anteroom in which all visitors must wash with a special soap and don sterile gowns. All visitors except his parents must wear a mask in Andre’s presence.

That does not keep them away, however. Andre’s father, Peter, and his two brothers, ages 5 and 10, visit frequently. The pastor of the Messiers’ church came down for a weekend to provide both Andre and the rest of the family support. Doctors and nurses have become Andre’s friends, and the hospital also provides “Child Life Specialists,” professionals at the business of keeping spirits up.

“These people come in, and (Andre knows) it’s just to have fun. They do everything they can to make their time with the child fun,” Peter Messier says. “I can’t say enough for the hospital.”

And then there are the surprises.

Last week, Andre received a letter from President Bush, who was responding to a letter he received from a friend of the Messiers. “The courage you have shown during this difficult time is an inspiration to all who know you,” the President wrote. “I was pleased to learn so many good things about you and I am happy to have you as one of my young friends.”

“Andre is a little on the shy side, but he’s very anxious to show people the letter when they ask about it,” Andre’s father says.

He showed a reporter from the Boston Sunday Herald the letter, and was rewarded with a story covering most of a page and headlined, “Brave boy fights for life.”

Andre has bad days, like Tuesday, when he was vomiting and feeling bad. Mrs. Messier’s conversation about her son was interrupted when his heart monitor began to beep. It was not a life-threatening moment, but it was a dip in the roller coaster of Andre’s days.

“When he’s really sick, he gets discouraged and wonders what is going to happen to him,” his father says.

But the last two weeks have been surprising and encouraging. Andre has felt good enough to play with his cars, and even try a little balloon volleyball. Always, his thoughts are on the end of October, when his treatment is scheduled to finish.

“He’s looking forward, I think the most, to seeing his friends, riding his bike in the driveway — just the freedom of it,” his father says. “He’s anxious to get home. We’re anxious to get him home.”