A Usual Lunacy — D.G. Compton

It makes people positively ache with happiness. It puts the roses back in their cheeks and the itch back in their blood. "It" is the Scholes Virus - proper medical term for what used to be called, out of mawkish ignorance but with uncanny prescience, the "love bug".

Professor Trevor Scholes has discovered, isolated and classified every variety of the infection that now bears his name. One variety, B79/K, is so rare that the odds are fifty thousand to one against two compatible carriers meeting.

So of course Giles Cranston and Tamsin McGillivray meet . . .