An excellent conceit - viral specialists being killed off by infectious letters. I wonder if the CIA got the idea for the anthrax attacks in November 2001 from watching this episode?
Unfortunately it turns a bit silly at the denouement, with people falling out of the nostrils of a giant display nose at a secret research facility. (And only an Avengersthon would require me to type a sentence like that.) And this sits very oddly with Steed's cold-blooded murder of the chief black hat: having safely bottled him up in a pit, he deliberately exposes him to the infection.
Very good sinister postman/chauffeur villain duo - chauffeur a young Dudley Sutton - quite a dangerous look he had back then, you can see why he was cast in Entertaining Mr Sloane.
I enjoyed Steed's speculation about the reverse selection process at the nursing academy, designed, he thinks, to choose all the most dishonest students.
Note the 4d sepia 1967 Machin definitives on the deadly envelopes. (Replaced in 1969 by the 4d vermillion as the sepia was too close in colour to the 5d dark blue). These would have been thought of as the new stamps at the time, having recently replaced the Wilding type seen in You Have Just Been Murdered. Interesting that though the villains are prepared to buy 10,000 envelopes from a high-class stationery firm, they're too mean to pay for first class 5d stamps.
Henry 'Benny Hill's straight man' McGee as the stationer btw.
Previously and also seen: Fulton 'Dr Quinn' Mackay returning from Return of the Cybernauts. He's wasted in this part though. Charles Lloyd Pack, who I think paints Six's portrait in It's Your Funeral, returning from Silent Dust. Hamilton 'Gen. Scobie' Dyce returning from Death on the Rocks.
Also seen: Valentine 'Black Guardian' Dyall as the butler.
Previously seen: Bruno Barnabe (third appearance). Geoffrey Chater from You Have Just Been Murdered. Andrew Laurence from Murdersville.
