SAN JOSE — In a stunning announcement that has South Bay mothers of newborns reeling, Santa Clara County officials on Friday said hundreds of babies, mothers and employees may have been exposed to a nurse with tuberculosis at Valley Medical Center’s mother and infant care center.

As a result, those who came into contact with the nurse from mid-August through mid-November will have to undergo testing for the potentially deadly disease. And as a precaution, all 350 infants born during that period also will be required to undergo a daily oral antibiotic treatment for six to nine months, whether or not they test positive, officials said.

“While the risk of actual infection is low, the consequences of tuberculosis infection in infants can be very severe,” Dr. Stephen Harris, head of pediatrics at VMC, said during a Friday afternoon teleconference.

He said the parents and other family members who may have been exposed are at much less risk than the babies and can be screened in a more routine fashion.

But Harris said the nurse in question — whom hospital officials did not identify for privacy reasons — “is not a very contagious individual” and did not report coughing or sneezing during work.

“It is still very unlikely that anybody will get infected from their contact with this employee,” the longtime pediatrician said.

TB is spread through the air from one person to another, public health experts say. The bacteria are released into the air when a person with active TB disease of the lungs or throat coughs or sneezes. People nearby may breathe in these bacteria and become infected.

Harris said the nurse had undergone her annual TB checkup in September and had tested negative. But in mid-November, he said, the nurse had sought medical care from her doctor for a reason unrelated to work. During a chest X-ray, her physician noted something suspicious in her lung, and she later tested positive for TB.

The nurse reported the finding to the hospital, and she was immediately placed on a paid leave of absence.

Since then, Harris said, all patients, visitors and employees who were potentially exposed have been identified, including 350 infants, 368 mothers and 338 employees.

Dr. Sara Cody, Santa Clara County’s health officer, said health officials contacted local and state infectious disease specialists seeking an opinion on a course of action and what time period the nurse might have been contagious.

Dr. Lisa Winston, a UC San Francisco professor of medicine and hospital epidemiologist at Zuckerberg San Francisco General Hospital and Trauma Center, said both tuberculosis skin testing and blood tests can be negative in people who have latent TB and people who have active TB.

“It is also possible the nurse contracted TB between September and November,” she said. “But given the short time frame, one might guess that a false negative test is more likely.’

Mothers who were in the hospital’s mother and infant care center during the three-month period are being contacted by phone and getting letters explaining what they need to do, Harris said. They can also call 408-885-3444 to set up their appointment.

Special appointments are being made for screening, testing and to provide medication. Medical staff are also available to answer questions, and all of the testing and treatments will be free.

The news unsettled mothers like Lidia Bermudez.

The 23-year-old San Jose woman gave birth to her first child, Joziah, on Sept. 2, and is scared not only that her son may have contracted TB, but also that he will have to endure months of medication.

“I want to get him in and checked out as soon as possible — that’s my main concern right now,” Bermudez said. “But the part that makes me worried is that he will have to take the medication regardless.”

Harris said he regrets the long period of medication for the babies, but he said that unlike older toddlers and adults — whose TB infection goes into the lungs and stays there — in infants the bacteria could enter their bloodstream.

“That is the danger,” Harris said, “because it can spread to other parts of the body.”

At the St. James Health Center in downtown San Jose, registered nurse Gabe Norwood said Friday afternoon that no mothers who may have been exposed had contacted the clinic yet.

“We’ve haven’t gotten any calls yet, but I’m sure we will,” he said. “I need to read more details about this event, but in general I’d tell them not to panic. There’s a low risk of infection.”

Early symptoms of TB include heavy coughing and coughing up blood, Norwood said.

The nurse said the clinic can test adults and children with a simple “finger prick” blood test to see if they are positive for TB. Getting back the results takes two days, he said.

While there is a vaccine for tuberculosis, the federal Centers for Disease Control and Prevention said it is not widely used in the U.S., but it is often given to infants and small children in other countries where TB is common.

Moreover, the vaccine, called Bacille Calmette-Guérin (BCG), does not always protect people from getting TB.

“TB is rare in the U.S., and the vaccine doesn’t work well,” said Dr. Niaz Banaei, a Stanford University expert in infectious diseases.

Staff writer Paul Rogers contributed to this report. Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.