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.