The HIV threat at Normandy High School has widened to include up to 50 students, health officials said Tuesday.
Health officials last week said "some" students may have been exposed to the virus that causes AIDS but recently refined their estimate to 50.
"It could well be substantially below that," said Craig LeFebvre, spokesman for the St. Louis County Department of Health. "Just because you're exposed doesn't mean you contracted the disease."
The health department hopes to open an on-campus clinic this week to offer free HIV tests to the school's 1,300 students, with testing to be completed by the end of the month.
The call for widespread testing at the school stemmed from a routine investigation into one HIV diagnosis that led to concerns about a number of at-risk students. Officials won't say whether the person who tested positive is a student.
When a new HIV case is identified it must be reported to a local or state health agency. Disease investigators interview the infected person to try to prevent the possible spread of the virus. The county has three investigators on its sexually transmitted disease team, LeFebvre said.
Investigators may not know the identities of all the students involved.
"If we knew who they were, we probably would have acted in a less public fashion," he said.
The most likely scenario for HIV exposure among teenagers would be sexual relations, but health experts say sharing contaminated needles for steroids, tattoos or drugs could also be the source.