You have several options, but it's very important that you try to do a few things:
- Minimize your presence. Specially with the client meeting, you don't want the client to remember your sickness as "the company is sick". It seems silly but it happens.
- Minimize your contact. You don't want to pass around your cold. So avoid shaking hands, etc. etc.
- Minimize your impact. Leave the room if your start coughing or sneezing. Try not to blow your nose, opt for 100 wipes instead of one nose blow.
- Minimize your recovery time. There will (hopefully) be action items from your meeting. Make sure you can get to them in a timely manor by minimizing your down time.
So what I would recommend:
Go to the meeting, and just the meeting. Show up for the "pre-meeting" if you guys do that kind of thing, but only go in for the meeting. If you can't call in or do a video conference, then fine, actually go into the office (I will assume you need to go in for the rest of the answer).
Sit away from everyone as much as you can. Take a seat in the back, by the door, make sure you have a trash can near by. Hide one under the table if you need to.
Try not to have to be the active speaker. If you need to keep it short and sweet. Cut out the manager speak as much as you can. "Yes, we can do that", "No, that's not doable.", "Let me look into that." Try and stick with safe answers like that.
Try not to have to commit, or make any on the spot decisions. "I will look into it and get back to you." is much safer then committing to something because you are tired.
Bring lots of bottled water. Usually, we feel bad when were sick because were dehydrated. All that extra snot production takes a lot outta ya. DRINK A LOT OF WATER before the meeting. You will look and feel better. You won't actually be better, but you will be able to fake it better.
Take notes, or record the meeting. Again you are not thinking straight, so make sure you can review something when you are back on top of your game.
When the meeting is over go home. Don't hang around, don't chat, don't go to lunch. Go home. Focus on recovery. This is specially important for the first meeting as your going to have to "fake it" the second day too.
At home things:
Don't take medicine that will make you sleepy. Do take medicine that has a decongestant.
Don't drink stuff with a lot of sugar or caffeine. Do drink a lot of water . Do not alter your drinking/eating habits more then you need to. For example if you have a cup of coffee in the morning. Drink that same cup of coffee. Now is not the time to change that.
Don't "not eat", even with a stomach flu. Do eat foods that are high in carbs and starches. Eat a whole pound of bread if you want. You body will need the extra energy, and the starch will turn to sugar at a much even rate then sugar, giving you a kind of "long burn" on your energy.
Do not stay up, or sleep in. Do get up at a normal time. Sleeping too much will make your groggy, too little and your will end up snoring and sneezing during the meeting. Your body needs sleep, but get it after the meeting, not before. Specially in the time between meetings. Sleep a lot then.