(CNN) -- All 1,800 students and staff at a southern California high school will be screened for tuberculosis Friday after 45 students tested positive for possible exposure, authorities said.
The Riverside County Department of Public Health and state officials have determined there is a possibility of exposure to other Indio High School students, though "the risk of transmission appears to be moderately low," according to a letter to parents on the school's website
Officials said there is one active case of tuberculosis and they do not fear an outbreak.
"The entire school is being tested out of an abundance of caution," Riverside County Public Health Officer Dr. Cameron Kaiser
Students will have to return to school in Indio, California, on Monday to have the test results evaluated, the letter says. Without verification of a current TB testing and results, students won't be allowed to return to school after the holiday break, on January 6, the school said.
County health officials expanded the testing to the entire school "after results of an initial screening were higher than expected," spokesman Jose Arballo Jr. said in a statement.
Officials tested 131 students on Monday, and 45 of them tested positive to possible exposure to the illness, he said.
"Someone who is exposed does not necessarily have active tuberculosis, a condition that must be confirmed with more tests," Arballo said. "On Thursday, follow up X-rays identified five students who require further examination."
Tuberculosis is an infection caused by a bacterium called Mycobacterium tuberculosis that mainly affects the lungs. It can also infect other parts of the body including the kidneys, spine and brain. TB spreads through the air when a person with an active TB infection coughs, sneezes, speaks or sings. Germs can stay in the air for hours. It cannot be spread through handshakes, sharing food or drink, or kissing.
According to the Centers for Disease Control and Prevention, approximately one-third of the world's 6 billion people are infected with TB.
Treatment of TB requires antibiotics for at least six to nine months, the Mayo Clinic says on its website.