Scientists believe there are at least two different strains of the COVID-19 virus causing illnesses.

A preliminary study conducted at Peking University’s School of Life Sciences and the Institute Pasteur of Shanghai discovered that one strain of coronavirus, type L, accounted for about 70% of the cases in China and is much more aggressive than the other strain, type S, which is milder and is the source of about 30% of the infections.

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The coronavirus outbreak began in Wuhan, China, and has since spread to dozens of countries, infecting almost 97,000 and killing at least 3,305. Researchers said type L was more common in the “early stages of the outbreak in Wuhan” but began to decrease “after early January 2020.”

“Human intervention may have placed more severe selective pressure on the L type, which might be more aggressive and spread more quickly. On the other hand, the S type, which is evolutionarily older and less aggressive, might have increased in relative frequency due to relatively weaker selective pressure,” the scientists wrote.

The researchers said the S type is thought to be the ancestral strain and that the findings “strongly support an urgent need for further immediate, comprehensive studies that combine genomic data, epidemiological data, and chart records of the clinical symptoms of patients with coronavirus disease 2019.”

The genomes studied were only sourced from China, so it is unclear what strains are most common in other countries or if there have been further mutations.

Cases of the illness in the United States have shot up in recent days. As of Thursday afternoon, there were 162 confirmed cases and 11 deaths.