Researchers say that malaria strain P.knowlesi, thus far found in monkeys only, is spreading to humans too.
Of particular concern is that this strain could be confused with another parasite P.malariae, found in tropical and sub-tropical regions across the globe. The latter is considered considerably less serious.
A study at the University Malaysia Sarawak shows that P. knowlesi can easily be mistaken for P. malariae under the microscope.
However, unlike its cousin, P. knowlesi has the ability to reproduce every 24 hours in the blood - meaning infection is potentially deadly.
Researcher Professor Balbir Singh said this meant early diagnosis and treatment were crucial.
The researchers carried out tests on over 150 patients admitted to hospital in Sarawak, Malaysian Borneo, between July 2006 and January 2008 with malaria infection.
They found that P. knowlesi accounted for more than two-thirds of the infections, resulting in a wide spectrum of disease.
Most cases of infection were uncomplicated and easily treated with drugs, including chloroquine and primaquine.
However, around one in ten patients had developed complications, such as breathing difficulties and kidney problems, and two died.
Although the fatality rate was just under 2%, that made P. knowlesi as deadly as P. falciparum malaria.
And the researchers stress it is hard to determine an accurate fatality rate given the small number of cases so far studied.
All of the P. knowlesi patients had a low blood platelet count, significantly lower than that usually found for other types of malaria.
However, even though blood platelets are essential for blood clotting, no cases of excessive bleeding or problems with clotting were identified.
The researchers believe the low blood platelet count could be used as a potential way to diagnose P. knowlesi infections.
Professor Singh said: "The increase in tourism in South East Asia may mean that more cases are detected in the future, including in Western countries.
"Clinicians assessing a patient who has visited an area with known or possible P. knowlesi transmission should be aware of the diagnosis, clinical manifestations, and rapid and potentially serious course of P. knowlesi malaria."
Although the new form of the disease has so far been concentrated in South East Asia, the researchers warn that tourism to the region could soon see cases appearing in Western countries too.
The work appears in the journal Clinical Infectious Diseases.