'Gestation' refers to the carrying of an embryo, not of an infectious disease!
You seem to be referring to the 'incubation period', which most definitely varies. See here:
http://www.nhs.uk/chq/Pages/1064.aspx?CategoryID=200&SubCategoryID=2001
However some diseases have far shorter incubation periods than those shown there. For seasonal influenza, for example, it's 1 to 4 days, with symptoms often appearing 2 to 3 days after infection. For the a common cold the incubation period is often around 2 days.
Many forms of food poisoning can result in symptoms, such as vomiting and diarrhoea, appearing within hours. If you have contact with a sufferer and their hygiene standards aren't high (e.g. if they prepare food for you without washing their hands thoroughly) you might well go down with food poisoning in well under a day.