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Their vaccine is made by inactivating influenza viruses with a carcinogenic chemical called beta-propiolactone.

That means the tests could be picking up inactivated fragments of the virus, like those found in commercial pasteurized milk.

Pasteurization and proper cooking techniques inactivate the virus.

However, when further testing was conducted — virus isolation — they were able to show it was inactivated virus that had been denatured by heat.

As a result, tortoiseshell cats end up with separate patches of black and orange fur depending on which chromosome was inactivated in that part of their skin.