If Russia had a secret sci-fi weapon like this it would be very stupid (and completely useless) to waste it on US-diplomats.

If it exists, it is likely is an unintended side effect. Either something is more toxic than expected, more contaminated, or more physically damaging to people. Or the effect is quite common if one just looks closely at (a specific group of) people, because „healthy“ just means „not properly diagnosed, yet“.