You've probably heard that to keep up in the business world you need to be tough and never show weakness. Especially if you’re a woman. You’ve got to hide all weakness to keep up with the fellas, right?

I call hogwash.

I used to think this way, too. I would force myself to work with a fever. You know what? My work suffered and my clients noticed. It is much more professional to give yourself plenty of cushion when it comes to deadlines so you can give yourself a break.

If you don't have any wiggle room, be honest. Say to your editor or client, "Here's the assignment. I've been under the weather, so if you see anything that's off, I apologize. Let me know and I will fix it ASAP. " I've used this exact line. Clients don't get mad. They understand you're human. They appreciate that you worked hard for them through your sickness and that you're honest and willing to make corrections right away.
There are a few times that I've made colossal blunders while sick or while in a stressful family emergency. It's better that the client know you're compromised, not incompetent.

By the way, it's also much better that the client knows right off. If you tell them after they find a problem it will sound like you're making up an excuse, which WILL look unprofessional.
And hey, if your client loves the work you did and there's no problem they will see you as a superstar that can get sh*t done even when you're sick. Win-win.

There will be the few jerks who will reprimand you or give you attitude for being sick. To that I ask, “Do you really need people like that as clients?” Probably not, because those types of people tend to love to pay you very little and expect way too much.

Have you ever had to write while sick?