Each year malaria kills about 400,000 people, most of them children and pregnant women in Africa. Scientists have long known that most of those fevers and deaths occur during the rainy months, when mosquitoes abound. But how does the disease persist during the long dry seasons, when almost no one falls ill and there are few mosquitoes to carry the tiny malaria parasite from one human host to another?

A new study in Nature Medicine by researchers from Germany and Mali has provided at least a partial answer: The parasite enacts a genetic change that enables it to hide in a person’s bloodstream for months.

The researchers began by drawing blood at regular intervals from almost 600 children and young adults in Kalifabougou, a town in rural Mali. Blood tests revealed that, even when samples had too few parasites to be seen under a microscope, about 20% of the participants still had very low levels of parasites hiding inside some of their red blood cells.

In some infected cells, the parasite produces sticky proteins that rise to the surface of the cells. These cells adhere to the walls of the veins and arteries, instead of being swept into the spleen, to be destroyed. The new study found that this activity varies with the season.

The spleen is something like a sieve, through which only young, flexible red blood cells can squeeze, said Silvia Portugal, a malaria specialist.

Stiff, older cells, as well as cells that are packed with multiplying parasites, are typically caught in the spleen and digested by large white blood cells called macrophages.

In much of Africa, malaria parasites are abundant in the rainy season, causing red blood cells to pump out sticky proteins, cells can jam the tiny capillaries in the brain.

Each parasite has a long menu of proteins written into its genes, from which it can order selectively. It can produce up to 60 variants of the proteins that migrate to the surface. Normally, a parasite will shift to a new protein every few days.

But during the dry season, the researchers found, the parasites in most red blood cells stopped making the sticky versions. They slipped away into the spleen to their destruction. But a few clingy survivors hung on, and appeared to slow down their metabolism, like microscopic bears hibernating for the winter.