russian flu
A rare from of flu that starts off like a common cold, but you still end up dead. Then it turns out you had been poisoned with Polonium or some sort of neurotoxin.
Brian: "Have you heard about that spy who allegedly got poisoned in England?"
Sam: "Eh, he just got the russian flu, nothing out of the ordinary."
Sam: "Eh, he just got the russian flu, nothing out of the ordinary."