For the first time in 10 years, global malaria cases are no longer falling, sparking concerns about a resurgence of the too often deadly disease.
Experts, and philanthropist Bill Gates, are urging country leaders gathering at the Commonwealth summit in London to pledge more money to fight the illness.
In 2016, nearly half of the world's population was at risk of malaria.
There were 216 million cases of malaria in 91 countries, an increase of five million compared with 2015.
Without more investment and preventive action, we could see even more malaria and rising deaths, experts warn.
Malaria cases have been increasing in some parts of the Americas, South-East Asia, Western Pacific and Africa, although in many other regions infections are stable or going down.
Part of the problem is the mosquitoes and the parasites that cause and spread malaria are developing resistance to the weapons we use to fight them - insecticides and antimalarial drugs.
New treatments are on the horizon.
An experimental new vaccine, Mosquirix, is already being used to protect young children in selected areas of Africa, which is home to 90% of malaria cases and 91% of malaria deaths.
But developments take time and money. And global funding to combat malaria has plateaued.
A new commitment to halve malaria cases over the next five years could save 650,000 lives across the Commonwealth, according to experts.
The UK government has said it will invest £500m a year for the next three years to help fight malaria. The Bill & Melinda Gates Foundation will extend its investments in malaria by an additional $1bn (£700m) through to 2023.
Prime Minister Theresa May said: "The job is not yet done. Today there are millions still at risk, economies held back and a child's life needlessly taken every two minutes from this disease. This is why I am championing a new Commonwealth commitment to halve malaria across member countries by 2023."
James Whiting, from Malaria No More UK, said: "The malaria campaign is at a crossroads.
"It is a disease that has probably killed more human beings than any in history.
"Malaria is not something that stands still."