Jerome Smith said he never got tested for HIV until it was almost too late. By the time he was checked five years ago, his immune system was so weakened that he had developed AIDS. The doctor told Smith, a D.C. government worker, that he had probably been infected for at least a decade.

Smith, now 39, says he doesn’t understand how his disease had been missed. In the two years before the diagnosis, he had been treated in local hospital emergency rooms for pneumonia, for unexplained fever and for a deadly bacterial infection. Each time, he underwent countless tests as doctors tried to figure out why he was sick. Not once, he said, did anyone test for HIV.

“They kept taking my blood and telling me they were going to see what’s wrong,” Smith said. “I thought they were automatically testing me for everything.”

As the International AIDS Conference gets underway in Washington this week, organizers are calling for big increases in HIV testing and treatment. Testing-and-treating is now the central AIDS prevention strategy in the United States, providing a critical tool beyond condoms, behavior change, clean hypodermic needles and a safe blood supply. Experts say treatment-as-prevention has the potential to dramatically alter the landscape of the epidemic. Not only does treatment improve a person’s health, but people treated with antiretroviral therapy can prevent the virus from being transmitted to others. The drugs prevent the virus from replicating, reducing the level of active virus in blood and other bodily fluids that are the usual vehicles for infection.

“While we undoubtedly still need a cure and a vaccine, we can save millions of lives with the knowledge we have today if we fully implement the proven strategies we now have to treat those living with HIV and prevent new infections,” said Diane Havlir, the U.S. co-chair of the conference.

Yet about half of Americans, 51 percent, don’t know that treatment helps prevent the spread of the disease, according to a new poll by the Washington Post and The Kaiser Family Foundation.

Following new national recommendations on HIV treatment, the District announced last month that health-care providers will start treatment immediately for all those newly diagnosed instead of waiting for evidence that someone’s immune system has been severely damaged.

The District has some of the most aggressive and innovative testing programs in the country — including free HIV testing at a Department of Motor Vehicles branch, at an Anacostia social services center and more than 20 other sites around the city. But still, cases such as Smith’s point to the many practical, institutional and cultural reasons that keep many from being tested.

Nearly 20 percent of Americans with HIV don’t know they are infected according to the U.S. Centers for Disease Control and Prevention.

In the nation’s capital, which has a higher HIV infection rate than any state and among the highest for cities, there are 14,465 adolescents and adults, or about 2.7 percent of the population, diagnosed as living with HIV. That is well above the 1 percent considered by the World Health Organization to be a general epidemic.

In addition to those diagnosed cases, another 4,000 to 5,000 people “are walking around the District of Columbia and they don’t know they’re positive,” said Gregory Pappas, senior deputy director at the District’s health department.

This group of HIV-positive people will be responsible for about 50 percent of all new infections, Pappas said.

Since 2006, the District has had an “opt-out” policy, meaning patients are supposed to be given HIV tests unless they say they don’t want one. But many doctors still mistakenly believe they must first get patient permission.

“Here we are in 2012, with a treatable, preventable disease, and we’re still asking for people’s permission to test for HIV when we don’t ask them if they want to be tested for cholesterol or heart disease or diabetes,” said Lisa Fitzpatrick, who heads the infectious disease care center at United Medical Center, a collaboration with Howard University.

In addition, a significant number of District residents are not diagnosed until late in their infection, when therapy may not be as effective. Without treatment, it takes an average of 10 years for someone with HIV to develop AIDS.

In the District, the total number of newly diagnosed HIV and AIDS cases dropped slightly in 2010 to 835 from the previous year. But what concerns experts are the large numbers of new patients whose infections have advanced to AIDS.

Doctors and public health officials say one way to address the problem is to make HIV testing routine.

But studies and anecdotal evidence point to a gap in understanding among doctors. A 2011 report by the Institute of Medicine found that the limited education, training and resources of health-care providers make it difficult to incorporate routine testing into their practices.

Health department surveys of high-risk populations show that about 70 percent of people who are recently diagnosed as HIV-positive in the District had gone to a doctor within the previous year but weren’t offered the test, Pappas said.

Too often, Fitzpatrick said, physicians’ mindsets are stuck in the 1980s. “They don’t test their married people. . . . They’re very uncomfortable talking about sex. We need to integrate sexual history, just as simple as information about their risk factors for heart disease.”

One woman who was worried about her weight loss asked her doctor whether it might be HIV. “ ‘He said, ‘You’re 56, you’re married, why do you need to be tested?’ ” recalled Daphne Rogers, a community health worker who counseled the distraught woman after she found out six weeks ago that her infection had progressed to AIDS.

To increase clinician knowledge, D.C. Council member David A. Catania (I-At-Large) authored legislation, passed in May, that incorporates HIV/AIDS education into requirements for medical professionals’ license renewal.

Most insurance plans are supposed to cover HIV screening in routine medical costs, health officials said. However, hospital emergency rooms usually do not, because a person arrives with a specific condition that may not be related to HIV.

The District is the only jurisdiction that enacted a law requiring insurers to cover the cost of an HIV test in emergency rooms. Since 2006, the D.C. health department has also provided free test kits and funding for staff to perform HIV tests in emergency rooms.

In practice, however, some insurers may not reimburse doctors if patients show no symptoms of the disease, doctors said. Doctors who are not specialists may also shy away from the responsibility of treating HIV-positive patients.

“Physicians know that if someone is HIV-positive, then it’s a lot to take on from a patient perspective,” said Gilbert Daniel, an internist in Southeast Washington who tests patients routinely for HIV. Last year, he discovered a 71-year-old patient was infected with HIV. “This was just someone who was coughing,” Daniel said. “This person had never been tested before.”

At George Washington University Hospital, officials have tried to bill insurance companies for HIV testing. But the two sides are in a dispute about the level of reimbursement, health officials said.

Why don’t more patients ask for the test?

“It’s stigma in spades,” said Justin Goforth, director of medical adherence at the Whitman-Walker clinic in Northwest Washington.

Despite medical advances that allow people living with HIV to take medicine and live a normal life, he said, many people only remember the dire images from the epidemic’s early years: toxic medications, discrimination and death.

After Smith’s HIV infection was diagnosed, he began immediate treatment. But the drugs did not seem to work. By the end of 2010, he was in hospice care and blind, a common complication early in the epidemic but one that is rare now. He was told he had three weeks to live.

Then, a hospice nurse convinced Smith to see John Bartlett, a leading AIDS specialist at Johns Hopkins. Bartlett changed Smith’s drugs. His viral load plummeted. By mid-2011, the virus was no longer detectable. Smith gained 90 pounds.

“His weight went up, and his smile went up,” Bartlett recalled. “He was one of the most rewarding patients I’ve ever had.”

Smith now gets treated at the Whitman-Walker clinic. His drug regimen is the same: four pills twice a day. Doctors were not able to save his eyesight, so he is now learning new skills including how to cook and use the computer. He soon hopes to find a new job in customer service.