MURRIETA, Calif. -

Riverside County health officials were assisting staff at Vista Murrieta High School on Wednesday to ascertain how many students and teachers may have had contact with a student recently diagnosed with tuberculosis.

``We're conducting an investigation to determine how many people may have been exposed to this person,'' county Department of Public Health Communicable Diseases Specialist Barbara Cole told City News Service. ``As soon as we know who they are, we'll be sending out notices for them to be checked.''

Cole said the department was notified Monday that a Vista Murrieta student had tested positive for active TB.

``The teenager has been placed in home isolation and is receiving treatment,'' she said. ``Generally speaking, a patient with this type of infection is treated with antibiotics for up to nine months, though they don't stay in isolation the whole time.''

The student's identity was not disclosed. The youth's infection does not appear to be connected to another TB case at the school in October, according to Cole.

Murrieta Valley Unified School District spokeswoman Karen Parris told CNS that the teen in the earlier case is ``doing just fine and out of danger.'' She would not confirm whether the youth had returned to school.

The school district has sent notices to parents and guardians alerting them to the latest TB case. Cole said the list of individuals likely to have come into contact with the infected student will be narrowed, after which the health department will send out letters advising that they receive skin tests, which should reveal the presence of TB.

``Most people who share the air and have casual contact with an infected person don't become infected themselves,'' Cole told CNS. ``It's only a small percent whose dormant germs end up active. But that's usually from ongoing exposure.''

To date, 13 active TB cases have been recorded in the county, health officials said. In 2013, there were 54 confirmed infections, including one at Indio High School. That case led to more than 1,300 students and staff being screened, a number of whom tested positive for TB exposure, though none apparently came down with symptoms.

According to the U.S. Centers for Disease Control, tuberculosis is spread through coughing, sneezing, singing or speaking. People cannot be infected through hand-shaking, kissing or handling bedding and toilet seats, according to the CDC.

Health officials noted that some people can be infected with TB without manifesting symptoms, which include fever, coughing, night sweats and chest pain.

Those with inactive TB are generally not infectious.