ANTIOCH -- About 200 Deer Valley High School students will be tested for tuberculosis later this week after one of their classmates was diagnosed with the infection -- the first case on a Contra Costa school campus in four years.

Contra Costa Health Services will be at the southeast Antioch school Thursday to test any student who shared a classroom or was in a club with the student with tuberculosis, Principal Kenneth Gardner wrote in a letter to parents last week.

The school is not recommending all students or staff get tested at this time, Gardner said. It is uncertain how many will be tested, but the parents of 209 students were sent consent forms, he said.

The Deer Valley student who contracted the disease is no longer infectious and was back in school Monday, county communicable disease Chief Erika Jenssen said.

The county found out on March 19 about the sick child, whose diagnosis was confirmed six days later, Jenssen said. After an investigation, it was determined on April 1 that parents should be alerted about the possible exposure. By then, however, Deer Valley was on spring break.

The student could have been infectious at school starting early January, according to county health officials. And, although the risk of progressing from TB infection to TB disease is only about 10 percent, it takes at least three months for that to happen, officials said, noting students will be offered tests again in May.

Tuberculosis is an airborne bacterial infection of the lungs spread through the air when a person with the illness coughs, sneezes, shouts or talks. Symptoms typically include a prolonged cough, unintentional weight loss of at least 10 pounds, fevers and night sweats.

The blood tests will be the first at a Contra Costa school campus since a student at Antioch High had tuberculosis in 2009, Jenssen said.

"It's not usual," she said.

The county also conducts tests at the workplaces and other environments where it is contracted, Jenssen said. "Every situation is somewhat different, but there is always an investigation," she said.

A recently released report found that there were 55 cases of tuberculosis in Contra Costa County last year, down dramatically from the 105 cases reported in 2001, but about the same as the past couple years.

Of those 55 cases, only three of those infected were between the ages of 15 and 24.

For more information about tuberculosis, go to http://cchealth.org or call 925-313-6740.

Contact Paul Burgarino at 925-779-7164. Follow him at Twitter.com/paulburgarino.