Ask yourself this question: "Does my illness impact my ability to do this job?" If it does, then you're going to need to discuss some aspect of this sooner or later to ensure that you can do the work. If it doesn't, then it depends upon your emotional needs to have other people know about this facet of your life. If it doesn't impact your work life now but becomes a problem later, discuss it then. Be sure to make it clear that this wasn't the case when you were hired.

2 | What is the risk of not telling them during the interview process (ex. They think I am deceptive.)

If you know that your chronic illness does impact your work life (i.e. doctors appointments, need specific physical accomm-odations at work, extended periods away from work, etc.), you could create resentment once you're on the job and then disclose. Your boss might consider you untrustworthy and you will have to hard to work to prove that you are to be trusted. You also don't get the opportunity to assess how "employee friendly" this organization really is. Do you want to work someplace where your personal needs are seen as a burden, rather than a matter of course?
Whereas, if you do disclose during the interview, you're all on the "same page" at the time of hiring; an employer will recognize that your disclosure shows your interest in being a solid member of the team.

3 | If I don't tell an employer about my illness during the interview, and then am hired, when do I explain and what is the best way to go about it?

It's your responsibility to accomplish your job. When your illness is preventing you from getting your job done and you need something from your employer to enable you to get the job done, that's the time to talk about it. Be matter of fact: "This is the situation and this is what I need in order to do my job." Emphasize that you can do the job with minor accommodations.

4 | If I end up not being able to work the expected amount of time (such as 40 hours) due to my illness, but financially need to be working, at what point should I resign, or should I just wait until I am let go? What's worse for my "record" that will affect future job possibilities?

Note: this can also be an "ADA" issue - there are laws protecting workers who become ill while on the job - the employer has to make accommodations. This is not my area of expertise, but these are my suggestions:
This is not a one size fits all situation. The best option would be to find a way to do the job in a more flexible manner that fits your needs. Think creatively, prepare the options, and go to your employer with some solutions, so it doesn't become his/her headache. If your employer won't be flexible, don't work past your effectiveness, because the worst outcome is to lose your job (for your self-esteem and for your job record). All experts say it's better to look for work while employed. And, it's better to leave a job when your employer still thinks well of you. But, in reality, sometimes you just can't find something else and you need that paycheck.

5 | Any other advice?

If you choose to disclose during the interview process, don't make that the first or even second thing you talk about. Why? You're interviewing because you believe you can do the job. Your illness is a secondary issue. Wait until they know you, want to hire you and you are in the negotiating phase. Then describe it in "matter of fact" terms, stating how it this impacts your work life and what you do to ensure it doesn't get in anyone's way. =

Rosalind Joffe has lived 25 years with MS and is the Founder of CIcoach.com, where she coaches people with chronic illness on how to thrive in the workplace. Sign up for her free newsletter for helpful information and "news" about illness in the workplace or free tele-seminar events. Call 617-969-1930.