Actions while "Bedridden"

I've been looking at rules for mortal wounds and the giant rat disease ability. I feel certain that my players will attempt to fight through their bedridden-ness. What should I say if a player wants to attack or take actions from which he is prohibited by injury or disease? He just can't muster the strength to move his weapon? His legs simply will not carry him faster than half his speed?

Also, I want to know if I should apply the giant rat disease immediately upon contraction (i.e. PC falls horribly ill within 10 seconds of being bitten by the rat, cannot attack or effectively flee because his speed is halved), or give it a gestation period before the PC suffers the penalty. The first option sounds a little bit brutal, so I just want to double-check if that's the intended effect.

I'd go with an incubation period to be sure, say about a day or so. Movement cut in half sounds fine and as for attacking while violently ill, I'm partial to attacks only succeeding on a natural twenty with no extra bonuses that would normally come from such a result. Alternatively, let them act normally at the cost of 1 or more hit points per round of forbidden action.

I think it depends on the fiction of what's slowing them down. Weakness could be an explanation, or pain, coughing fits, hazy senses, disorientation, loss of balance... this is starting to sound like the side effects of major prescription medicine! I'll usually be up-front with my player - their character can't perform useful actions right now.  It's the perfect time for a henchman to step up as the player's acting character.

I tend to apply mundane disease effects the morning after the disease is contracted, but my magical diseases apply immediately.