It can be very hard to work on establishing a new habit when one is sick. Being positive might seem like a good idea, but there is usually another part of oneself that says, "I have a right to be miserable!" If your new habit is something like exercise, it can be hard to tell the difference between really needing to rest and using being sick for an excuse.
Here is one important thing to remember. In order to truly establish a habit, you can't regularly take days off during the 30 day commitment period. No one is perfect, so yes, you may miss a day, but that means that you absolutely shouldn't miss the next day! Missing two days could lead to you needing to start all over.
It is all fine to say that you should never miss a day you might be thinking, but what if I am really sick! Well, there are things that you can do to keep establishing a habit without compromising your sick body. For example, I am sick now. I am not horribly ill, but I have a cold that includes a headache, a sort throat, and a low grade fever. I need extra rest. I don't want to stop my walking habit though. So I walk slower or walk for less time. I stay closer to home in case I feel I should stop. If I was even sicker, I might modify further. I could see doing some simple stretching exercises or getting up and walking around the house during exercise time if I was very sick. Even meditation could be substituted for walking.
My point is that if illness strikes, keep establishing that habit, but use modification if necessary. Be honest with yourself about what you can and can't do. Talk to your support person. Make sure that you stay on track, and at the end of the month, you will be happy to have a new habit that will stick by you in sickness and in health.