For 10-year-old Jacob Krause, getting ready for the new school year wasn't a simple matter of back-to-school shopping. It also involved working out logistics for getting to the bathroom as many as 20 times during a single school day.

The Clarksville Elementary School fifth-grader has severe ulcerative colitis, an inflammatory bowel disease that increasingly, and somewhat mysteriously, strikes children.

The number of children afflicted by colitis and another inflammatory bowel disease, Crohn's disease, has increased 50 percent in the past decade, according to the Crohn's & Colitis Foundation of America. About 1.5 million Americans suffer from colitis and Crohn's, about 10 percent of them under the age of 18.

"We're seeing younger and younger children getting it over time," said Dr. Maria Oliva-Hemker, chief of the Johns Hopkins University's School of Medicine's division of pediatric gastroenterology and nutrition.

The reasons for the increase are not clear. But many researchers believe something in the environment must be behind the surge in pediatric colitis, Crohn's and other autoimmune diseases, which have been on the rise generally. One theory is that as the developed world has become more hygienic, the body has become less practiced at fighting off bugs — and more vulnerable to autoimmune diseases, in which the body attacks its own cells or tissues.

"Since we know [inflammatory bowel disease] is found in more developed countries, it must be something about the exposures that we are seeing in our day-to-day lives," Oliva-Hemker said. "Probably the fact that we are a more hygienic society, the fact that children, even at an early age, are kept in very clean environments — not necessarily outside, playing in the dirt, being exposed to very low levels of routine viruses."

If the home environment is "too clean early on," when children start school and are exposed to more bugs, "the immune system goes into overdrive," Oliva-Hemker said.

"As we've gotten rid of many infectious disease, the autoimmune and allergic diseases have steadily increased — lupus, allergies, eczema," she said. "And in countries where we still see a lot of infectious diseases, we don't see" autoimmune diseases.

Better awareness of the disease might also be leading to more diagnoses, said Karla Au Yeung, a Hopkins pediatric gastroenterologist who treats Jacob.

"GI in general for pediatrics didn't start until the mid-'80s," she said. "The practice is relatively new. I think people are more aware."

A difficult condition even for adults, inflammatory bowel disease poses special problems for young people who come down with it. One is the need for constant bathroom access — a particular challenge during recess and outdoor physical-education activities in an era when youngsters generally are not allowed to let themselves in and out of school buildings. There's also the likelihood that that disease itself, and some medicines used to treat it, will stunt growth. And there is a good chance that the condition will go undiagnosed for long stretches because many people don't realize it afflicts children.

"They just think it's an adult disease if they've heard of it at all," said Oliva-Hemker. "It's not that unusual of a GI condition, but a lot of folks feel it's something that's in the adult world and kids can't get it — even doctors. ... I can't tell you how many children I'm seeing and the diagnosis has been delayed."

There is no cure for inflammatory bowel disease. With colitis and Crohn's, the intestines are inflamed and sometimes pocked with ulcers. The colon alone is affected by colitis; with Crohn's, the inflammation can occur anywhere along the gastrointestinal tract, from the mouth on down. Abdominal pain, diarrhea and bloody stools are common symptoms, and if left unchecked, the condition can cause weight loss and malnutrition because the body cannot properly absorb nutrients.

There is a genetic component, particularly in cases in which inflammatory bowel disease afflicts young patients. Ten percent to 15 percent of adults who have the disease have a close family member who has it; the number is 30 percent for children with the disease. In Jacob's case, both his father and his maternal grandmother have inflammatory bowel disease. His father was diagnosed in 2004, at the age of 29; his grandmother, at age 18. Jacob was diagnosed at the end of third grade, as he was turning 9.

Whatever the cause, the disease can take a terrible toll on adults and children. Some patients can, with medication, keep the symptoms in check. But many suffer periodic "flares," in which gastrointestinal distress makes it difficult to work or go to school.

"When I have a really bad flare, I basically live in the bathroom that day," Jacob said.

Having kept a tally at his doctor's request, he recalled school days when he had to duck out of class 15 or 20 times to go to the bathroom.

Recess was a particular problem for Jacob last year because the school doors were locked when he was out playing. If Jacob needed to use the bathroom, he had to find a teacher outside, borrow her key and find a buddy to return the key to the teacher before he went inside. This year, Jacob's mother worked out a plan with his teacher that allows him to have his own key on a lanyard.

"If I have a flare, the teacher's gonna give me a key," said Jacob, who was blissfully free of flares all summer long.