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A new strain of bird flu, H7N9, has been discovered to be resistant to Tamiflu — the only treatment currently known for the disease. This wave of bird flu, declared to be “unusually dangerous” by the World Health Organization (WHO), appears to have mutated in a way that makes Tamiflu ineffective, Business Insider reports.

WHO’s influenza expert Keiji Fukuda said in a press conference in April that ”this is definitely one of the most lethal influenza viruses that we have seen so far.” The resistance against Tamiflu was discovered in Shanghai after sampling 11 infected people.