The Avengers.S06E04.You'll Catch Your Death

Empty envelopes are delivered to top Government officials, who are then found dead. The only clue is that each man seems to have died from a fit of sneezing. Steed investigates a clinic for researching the common cold, which seems to have purchased some unusual stationery supplies.
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Teleplay by Jeremy Burnham
Directed by Paul Dickson
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Guest Starring: Ronald Culver, Valentine Dyall, Fulton Mackay, Sylvia Kay, Dudley Sutton, Charles Lloyd-Pack, Henry McGee, Hamilton Dyce, Bruno Barnabe, Geoffrey Chater, Willoughby Gray