As in declare a sickness absence?
I've developed a bit of a cough. Obviously if I was expected in the office I couldn't go in, although I don't feel particularly ill. I'm also fairly autonomous so if I am feeling rougher in the morning I can probably just do the bare minimum and no one would notice for a day or two.
At what point do you declare yourself "sick" when you're not actually going in to the office? I.e. if I was well enough to keep on top of emails and do some routine tasks but not to complete something very demanding?