Three cases of tuberculosis (TB) were diagnosed among students at Robert E. Lee High School in Fairfax County, Va., prompting public health officials to offer free testing to hundreds more exposed to the deadly and highly infectious bacteria.

One student was diagnosed with tuberculosis in December 2012. This month, two more students were confirmed to have the disease.

In order to search for more undiagnosed cases, the school system sent letters to the families of 430 students and teachers offering free testing.

Caused by the bacterium Mycobacterium tuberculosis, the disease is highly contagious, spread by microscopic droplets released when an infected person coughs, speaks, or sneezes, according to the Centers for Disease Control and Prevention. Those most at risk are individuals who share enclosed spaces with an active TB infection.

Tuberculosis usually afflicts the lungs, but can attack any part of the body. TB is potentially fatal if left untreated. Symptoms include a cough, unintentional weight loss, and general malaise. More advanced cases involve chest pain, pain with breathing, and coughing up blood or sputum.

Gloria Addo-Ayensu, Fairfax County's health director, said that most of the 90 tuberculosis cases identified in the county last year originated from overseas. She added that most of the county's cases are highly susceptible to treatment.

Although there were less than 10,000 cases of TB in the U.S. last year, TB is an endemic problem worldwide. According to the World Health Organization, 8.7 million people acquired tuberculosis and 1.4 million died from the disease, making it one of the deadliest infectious diseases world wide, second only to HIV and AIDS. Infections due to strains of multiple-drug resistant tuberculosis, which are resistant to first-line drugs, have risen in recent decades. Reports of totally drug-resistant tuberculosis, a strain resistant to all current forms of treatment, first observed in Italy in 2003, has become cause for concern.