SAN DIEGO -- Doctors from Mexico and in San Diego are uniting to stop a dangerous, infectious disease from spreading.

10News followed some American doctors across the border to find out how a bi-national effort could impact the health of your family.

Tijuana -- Mexico’s fastest-growing city -- attracts millions of tourists every year.

Every day and night, tens of thousands travel across the busiest border crossing in the world. Many of those who cross take along with them a highly contagious disease -- tuberculosis.

“It doesn’t stop at the border. Nearly 25 percent of the cases in the U.S. are from Mexico,” a doctor at Tijuana General Hospital told 10News.

Tuberculosis is an airborne disease that is transmitted from one person to another and is a growing health concern in our border cities.

“San Diego and Tijuana have the highest concentrations of tuberculosis of all the border states,” said University of California, San Diego researcher Dr. Richard Garfein.

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The easy spread of the disease lies in the hard-to-reach, high-risk population.

Tijuana’s homeless population, sex-trade workers and drug abusers often do not know they are ill, or they wait until they are much sicker before getting help.

"We have patients that go into the U.S. They don't even know sometimes they are ill and transmit the infection to people on the other side of the border," said Garfein.

That is why doctors from both sides of the border are uniting to control and prevent TB.

"With an airborne disease, people can come in contact even during a visit or standing in a line,” said Garfein.

The two-year project called Preve TB will bring doctors, nurses and treatment to the forgotten areas of Tijuana.

Garfein said, "I have seen there is really quite a bit of disease and a lot of poverty and a lot of circumstances that make it difficult to seek care.”

Garfein and his team of researchers travel across the border regularly to work with Tijuana doctors and the city’s at-risk population.

“We are trying to help to prevent infection and keep you from becoming infected, but also to learn more about protecting the community as a whole,” said Garfein.

Tammy De La Sierra, an American woman living in Tijuana, told 10News she is worried about her health because she frequently uses drugs.

“I’ve had hepatitis three times,” she said.

Every month, De La Sierra goes to a street corner where a staff of doctors and nurses working in a mobile van test her for TB and other diseases.

One homeless man was concerned about contracting tuberculosis again.

"I had the test five years ago and I turned out positive," the man told 10News.

Garfein said, "We have already tested over 700 people, and we found that over 70 percent are infected with tuberculosis."

Inside Tijuana General Hospital is the only laboratory in all of northwestern Mexico where doctors could test for TB with a simple blood test.

Garfein is working closely with Mexican pulmonologist Dr. Laniado-Laborin, who is using the new, state-of-the-art TB test.

"This test distinguishes perfectly between if you are infected or just vaccinated," said Garfein.

The traditional TB skin test is not accurate because it cannot distinguish between a true infection or a reaction from a vaccine widely used in Mexico.

"This is important because we treat those individuals for nine months with a drug that costs some money and it can be toxic," said Laniado-Laborin. "You will incubate the tubes for 24 hours and if the individual has been infected in the tube with the TB antigens, you will get a reaction that the machine will detect."

It is a reaction that will improve the lives of people who live, work and play on both sides of the border.

"Before, we were waiting for the patients to be treated. Now, we are going to go out and try to detect people that are infected and treat them before they get sick," said Laniado-Laborin.

Cases of TB in San Diego are showing a decline, according to recent research.

There were 305 reported cases of active TB in 2005.

However, what is concerning to medical experts is more than 70 percent of those cases originated from outside the U.S.

The highest number of TB cases from outside the U.S. originates in Mexico.