Why can't you have a vaccination if you have a cold?

From a medical perspective, what is it about a cold or fever that means you can't have a vaccination, for instance against influenza?

An immunisation works in this way:
Your immune system has a memory. I am going to explain this as if it were people running it- I think the analogy will help make it clear. If anyone wants it with all the anatomic and physiological words, ask again!

The Immune System is full of Soldiers. As soon as a potential bad guy enters town, they are alerted and immediately set out an ABP (all points bulletin) to hunt him down. When they catch him, they take pictures and fingerprints, make notes of what kind of soldiers he is weakest against (so they know the best weapon next time), and their generals make sure that every soldier, from that first encounter on, has knowledge of all the baddies. Then they make town uncomfortable- cranking up the heat (running a fever) and doing anything else they can to run the baddie out of town (like diarrhea when there is food poisoning).

This means that known baddies are on file: As soon as a known baddie is indentified as violating his restraining order, the immune system calls an alarm, the body wages an all out attack and the baddie is beaten before it even has a chance to set up camp.

The kind of baddie you are immunised with has either been killed, or weakened to an extent that it is not able to cause harm; your body doesn't know that and still identifies it as a bad guy for future reference. Thats why you often get a little sick unwell after an immunisation- its the remnants of the ward against the baddie- a good sign that the immunisation has worked well.

If you have a fever, when you get the immunisation then the baddie that was introduced to immunise you will be killed off before there is a chance to identify it, so it wont work. Your immune system doesn't have a way of saying "all of these guys should die... that one, keep him alive, I haven't seen him before!" Fever is very effective against germs of all shapes and sizes. Amazing theat when we get sick, we take stuff to stop a fever, huh? Thats a new question, isn't it!!!

Thats why you can't get an immunisation when you have a cold, flu or fever- the immunisation, in order to work, must have time to have the correct response and if the critters in the immunisation are killed off before they can be reacted to, then there is no benefit of the immunisation.

Hope that helps!

you can if all you have is a cold. they ask if you are sick with anything other than a cold. a fever means an infection, so they dont want to give you any injection at that time.

A vaccination is technically the doctors making you sick. They inject you with the disease (Don't, worry, not nearly enough to affect you) and your immune system fights it off immediatly (This is why you sometimes don't feel well a day or two after you get a flu shot). This way, if your body ever comes in contact with that disease again, your immune system knows how to fight it off.

Now, if you have a cold, your immune system is already working overtime to fight that off. So what happens if your immune system is distracted with curing your cold and all of a sudden you're injected with the flu? Your immune system is already busy, so you will get very sick, very quickly. Just wait until you are healthy and then go and get your vaccination.