The world’s most expensive drug, a treatment for spinal muscular atrophy made by Novartis, comes with a $2.1 million bill.

The gene therapy is one of a new class of drugs that can potentially cure intractable diseases by manipulating the genetics of patients. These treatments have given rise to a wave of enthusiasm — and general bafflement around how to pay for them.

It’s against this backdrop that the Bill & Melinda Gates Foundation and the National Institute of Health are committing $200 million over four years to fund gene-based cures for HIV and sickle cell disease that can be delivered at low cost to people in developing countries.

“In recent years, gene-based treatments have been groundbreaking for rare genetic disorders and infectious diseases,” Dr. Trevor Mundel, the president of the Gates Foundation’s global health program, said in a statement. “While these treatments are exciting, people in low- and middle-income countries do not have access to these breakthroughs. By working with the NIH and scientists across Africa, we aim to ensure these approaches will improve the lives of those most in need and bring the incredible promise of gene-based treatments to the world of public health.”

Over the past 20 years, the Gates Foundation has invested $10 billion into efforts aimed at fighting malaria, polio, the flu and solving sanitation problems, among other initiatives.

The two diseases disproportionately affect people in sub-Saharan Africa, which is home to 67 percent of all people with HIV. More than 11 million children will be born with sickle cell disease in the region over the next 30 years, NIH said.

“We are losing too much of Africa’s future to sickle cell disease and HIV,” said Matshidiso Rebecca Moeti, the regional director for Africa at the World Health Organization. “Beating these diseases will take new thinking and long-term commitment.”

One major scientific hurdle will be to find curative treatments that don’t look like today’s gene therapies, which involve drawing blood from patients, reengineering their cells, and reinfusing the blood — a costly process that requires extensive infrastructure. Instead, the organizations are hoping to create therapies that can be delivered directly into the body of the patient.

The organizations hope to identify promising treatments and fund them through clinical trials in the next decade.