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It begins with a mild fever and malaise, followed by a painful cough and shortness of breath. The infection prospers in crowds, spreading to people in close reach. Containing it requires contact tracing, isolation and treatment that can take months.

This insidious disease has touched every part of the globe. It is tuberculosis, the biggest infectious-disease killer worldwide, claiming 1.5 million lives each year.

Until this year, TB and its deadly allies, HIV and malaria, were on the run. The toll from each disease over the previous decade was at its nadir in 2018, the last year for which data are available.

Yet now, as the coronavirus pandemic spreads around the world, consuming global health resources, these perennially neglected adversaries are making a comeback.

“COVID-19 risks derailing all our efforts and taking us back to where we were 20 years ago,” said Dr. Pedro L. Alonso, the director of the World Health Organization’s global malaria program.

It’s not just that the coronavirus has diverted scientific attention from TB, HIV and malaria. The lockdowns, particularly across parts of Africa, Asia and Latin America, have raised insurmountable barriers to patients who must travel to obtain diagnoses or drugs. Fear of the coronavirus and the shuttering of clinics have kept away many patients struggling with HIV, TB and malaria, while restrictions on air and sea travel have severely limited delivery of medications to the hardest-hit regions.

About 80% of tuberculosis, HIV and malaria programs worldwide have reported disruptions in services, and 1 in 4 people living with HIV have reported problems with gaining access to medications, according to U.N. AIDS. Interruptions or delays in treatment may lead to drug resistance, already a formidable problem.

In India, home to about 27% of the world’s TB cases, diagnoses have dropped by nearly 75% since the pandemic began. In Russia, HIV clinics have been repurposed for coronavirus testing.

Malaria season has begun in West Africa, which has 90% of malaria deaths in the world, but the normal strategies for prevention — distribution of insecticide-treated bed nets and spraying with pesticides — have been curtailed.

According to one estimate, a three-month lockdown across different parts of the world and a gradual return to normal over 10 months could result in an additional 6.3 million cases of tuberculosis and 1.4 million deaths from it.

A six-month disruption of antiretroviral therapy may lead to more than 500,000 additional deaths from illnesses related to HIV, the WHO said. The WHO also predicted that in the worst-case scenario, deaths from malaria could double to 770,000 per year.

Public health experts warned that if the trends continue, the coronavirus is likely to set back years, perhaps decades, of painstaking progress against TB, HIV and malaria. The Global Fund, a public-private partnership, estimates that mitigating this damage will require at least $28.5 billion, a sum that is unlikely to materialize.

If history is any guide, the coronavirus’s impact on the poor will be felt long after the pandemic is over. The socioeconomic crisis in Eastern Europe in the early 1990s, for example, led to the highest rates in the world of a kind of TB that was resistant to multiple drugs.

The starting point in this ruinous chain of events is a failure to diagnose: The longer a person goes undiagnosed, and the later treatment begins, the more likely an infectious disease is to spread, sicken and kill.

“The more you leave undiagnosed and untreated, the more you will have next year and the year after,” said Dr. Lucica Ditiu, who heads the Stop TB Partnership, a consortium of 1,700 groups.

The infrastructure built to diagnose HIV and TB has been a boon for many countries grappling with the coronavirus. GeneXpert, the tool used to detect genetic material from the TB bacteria and from HIV, can also amplify RNA from the coronavirus. Now most clinics are using the machines only to look for the coronavirus.

In country after country, the pandemic has resulted in sharp drops in diagnoses of TB: a 70% decline in Indonesia, 50% in Mozambique and South Africa, and 20% in China, according to the WHO.

In late May in Mexico, as coronavirus infections climbed, TB diagnoses recorded by the government fell to 263 cases from 1,097 the same week last year.

The pandemic is also shrinking the supply of diagnostic tests for these killers as companies turn to coronavirus tests, which are much more lucrative, at about $10, compared with 18 cents for a rapid malaria test.

These companies “have tremendous demand for COVID right now,” said Dr. Madhukar Pai, the director of the McGill International TB Centre. “I can’t imagine diseases of poverty getting any attention in this space.”

According to the WHO, at least 121 countries have reported a drop in TB patients visiting clinics since the pandemic began.

“This is really difficult to digest,” Ditiu said. “It took a lot of work to arrive where we are. We were not at the peak of the mountain, but we were away from the base. But then an avalanche came and pushed us back to the bottom.”