Showing up to a job interview sick is going to panic the interviewer not impress them. So, if you have any respiratory or flu-like symptons, including a cough, runny nose, stuffy nose, or sore throat, do both yourself and the hiring manager a favor and stay home. You're going to be spreading unwanted germs, and besides that, you are not going to interview well if you're sick.
When You Can't Make the Interview
There are other reasons beside illness that necessitate rescheduling an interview. Most companies understand that circumstances come up i.e. a sick family member, a scheduling conflict, car troubles, or any one of a number of other reasons.
What's important is to let the company know in a timely manner that you won't be able to make the interview and to try to secure a new interview date when you talk to them.
How to Reschedule a Job Interview
The best way to reschedule a job interview is to both email and call the person who scheduled the interview. Let him or her know that you are ill and ask if it would be possible to reschedule. If a family member is ill or there's another reason you can't make it, let the interviewer know that, too.
Suggest a day or two when you'll be available if you email. If you call, ask the interviewer when would be a good time for you to interview.
When rescheduling because of illness, give yourself some time to recover.Give Notice
Either way, give the company as much notice as possible that you aren't going to be there. Being considerate of the interviewer's time will be appreciated. Cancelling at the last minute, unless it's a real emergency, may be held against you. So, contact the interviewer just as soon as you know you won't be able to make the interview.