Actually the WHO of the Title is the World Health Organization as well as the question as to which person is responsible for a small pox outbreak. Terry Thomas stars as the "germ detective" who stumbles upon a case of small pox on a flight into England. The man who has the disease is a recently married oil man who is returning to England and takes ill on the flight. Quarantined the doctors realize what the deal is and try to track down where he might have caught the disease. However things become complicated and potentially dangerous when two more people elsewhere in Europe come down with the disease. As Thomas tries to track down the common source of the infection (the disease takes two weeks to incubate so its very likely that all three victims have something in common) things become more complicated as people with questionable backgrounds and Arab ministers get brought into the mix.