Lisa Sterman holds up a Truvada pill at her office in San Francisco in 2012. The pill helps prevent HIV from infecting people. (Jeff Chiu/Associated Press)

Anthony S. Fauci is director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health.

In the summer of 1981, the world became aware of a mysterious new disease that was seen initially among a relatively small group of gay men in the United States and was soon shown to be caused by the human immunodeficiency virus. Fast- forward more than 30 years, and the entire world is struggling with one of the most devastating pandemics in history. More than 70 million infections have occurred, predominantly among heterosexuals in the developing world, resulting in more than 30 million deaths. Despite these horrendous statistics, advances in HIV treatment and prevention have transformed the lives of those HIV-infected people who have access to health care, and have provided us with highly effective methods of preventing HIV infection.

So why does this global pandemic continue to rage? It is not that we lack the medical advances and interventions to end the pandemic. It is that our proven tools have not been implemented adequately or uniformly.

Combination anti-HIV therapy became available in the mid-1990s, and although the drug regimens were highly effective in suppressing the virus to below detectable levels and allowed patients to live relatively healthy lives, some questioned whether the cumulative toxicities of long-term drug therapy would negate the beneficial effects over time. Controlled clinical trials have since put that concern to rest by showing that the mostly manageable toxicities of anti-HIV therapy are much less harmful than continued HIV replication in the absence of therapy.

Next, a groundbreaking study demonstrated that treating HIV-infected individuals sooner rather than later dramatically diminished the likelihood that they would infect their sexual partners. The public- health benefit of treatment for the prevention of further transmission was clear. Still, some argued that the health benefit to the infected person was unproven, putting clinicians in an unenviable position: They knew that infected individuals with uncontrolled virus could infect others, but they were unable to strongly recommend treatment for their patients. A study published last year put an end to this dilemma by showing that treating a person as soon as possible after diagnosis was much more beneficial than waiting until the person’s immune system showed damage. With these pivotal studies, there is now no excuse for delay; every person infected with HIV should be offered antiviral drugs upon diagnosis.

But doing so requires seeking out those at risk for infection and testing them; linking infected individuals to medical care; working to keep them in care; and providing anti-HIV drugs. It also requires careful attention to barriers to care such as poverty, substance abuse, and housing and food insecurity. Globally and domestically, we have not yet achieved this.

Most of this refers to those who are already HIV-infected. However, 2 million new infections occur globally each year, including 50,000 in the United States; the latter number has remained steady for almost two decades. The frustrating fact is that we know exactly who is infecting whom. Of the approximately 1.2 million people living with HIV in the United States, the 13 percent who do not know they are infected are responsible for transmitting about 30 percent of new infections per year. Even more striking, more than 60 percent of new HIV infections are transmitted by people who are aware they have HIV but who are not receiving appropriate care. Thus, if we identified all the infected people in the country and got them into continuous, effective care, including anti-HIV treatment, we could prevent more than 90 percent of new infections each year.

For uninfected, at-risk individuals, several modalities of prevention are available. An important recent advance comes from a series of clinical trials that convincingly demonstrated that regularly taking a single pill containing two anti-HIV drugs can reduce an individual’s risk of contracting HIV by more than 90 percent. Unfortunately, this prevention strategy — called pre-exposure prophylaxis, or PrEP — is vastly underutilized. The Centers for Disease Control and Prevention estimates that more than 1.2 million people in the United States are at substantial risk of HIV infection and could benefit from PrEP; however, less than 5 percent of these people are taking it. To make matters worse, one- third of primary- care doctors and nurses are unaware of PrEP and its potential health advantages. This must change.

Even in the absence of an effective HIV vaccine, which would be the final nail in the coffin of the pandemic, we have the tools to end the HIV/AIDS epidemic in the United States and globally. We can save the lives of infected individuals and prevent them from infecting others by getting them into treatment programs and maintaining them there. In addition, we can effectively prevent HIV infection in at-risk populations by a number of means, including the use of the highly effective PrEP.

It is often said that we were slow to recognize the seriousness of the emerging HIV pandemic during the early 1980s. At the time, our ability to fight its spread was meager. Today, we have the tools to end this modern-day plague. We must not squander the opportunity. History will judge us harshly if we do.