Wifey sneezed once around the middle of the afternoon and thought little about it. When she sneezed a second time an hour or so later, she felt a slight tickle. By early evening she had a streaming nose and a full blown cold.
No warning, no prior indication, nowt. Just from healthy to snot in a couple of hours.
Luckily we are expert at dealing with a cold. I put her to bed and fuck off up the pub to avoid the inevitable germies that I will eventually contract, thus also acquiring her sniffles. Except mine will be a fully blown bout of man flu and I shall have to stay in bed for a week at least.