What is it?  
Also known as the "Herxheimer Reaction", this reaction occurs when the body tries to eliminate toxins at a faster rate than they can be properly disposed of. The more toxic one's bodily systems are, the more severe the detoxification, or healing crisis. It is characterized by a temporary increase in symptoms during the cleansing or detox process which may be mild or severe. You may feel worse and therefore conclude that the treatment is not working. But these reactions are instead signs that the treatment is working and that your body is going through the process of cleaning itself of impurities, toxins and imbalances. 

Such reactions are temporary and can occur immediately -- or within several days, or even several weeks, of a detox. Symptoms usually pass within 1-3 days, but on rare occasions can last several weeks. If you are suffering from a major illness, the symptoms you experience during the healing crisis may be identical to the disease itself. Sometimes discomfort during the healing crisis is of greater intensity than when you were developing the chronic disease. This may explain why there may be a brief flare-up in one's condition. Often the crisis will come after you feel your very best. Most people feel somewhat ill during the first few days of a cleanse because it is at that point that your body dumps toxins into the blood stream for elimination. With a more serious condition there may be many small crises to go through before the final one is possible. In any case, a cleansing & purifying process is underway, and stored wastes are in a free-flowing state. 

Cause:
The healing crisis is the result of every body-system, in concert, working to eliminate waste products through all elimination channels and set the stage for regeneration. The end result: old tissues are replaced with new. When any treatment or cleansing program causes a large scale die-off of bacteria, a significant amount of endotoxins (toxins within the bacteria itself) are released into the body. The more bacteria present, and the stronger their endotoxins, the stronger the cleansing reaction. When any treatment or detox causes the organs of the body (particularly the liver, which is a storehouse of drug and poison residues) to release their stored poisons and toxins, a cleansing reaction may occur. Any program, such as fasting, which causes a rapid breakdown of fat cells (which are a storehouse for toxins), can cause a healing crisis as toxins previously lodged in the fat cells are released into the blood stream. 
 
Symptoms:
The healing crisis will usually bring about past conditions in whatever order the body is capable of handling at that time. People often forget the diseases or injuries they have had in the past, but are usually reminded during the crisis. There are a wide variety of reactions that may manifest during a healing crisis, the most common are: 

Increased joint or muscle pain

Diarrhea
Extreme fatigue and/or its opposite, restlessness
Cramps
Headache (believed to be caused by buildup of toxins in the blood)
Aches, Pains
Arthritic flair up
Insomnia
Nausea
Sinus congestion
Fever (usually low grade) and/or chills
Frequent urination and/or urinary tract discharges
Drop in blood pressure
Skin eruptions, including: boils, hives, and rashes.
Cold or flu-like symptoms
Strong emotions: anger, despair, sadness, fear, etc.
Suppressed memories arise
Anxiety
Mood swings
New phobias develop

• If you are feeling fatigued, or sleepy, your body is talking to you, and telling you to rest. Be kind to yourself, and get the rest that you need.
• Symptoms frequently disappear immediately after a good bowel movement, use an enema to provide relief. 
• For other symptoms, meditation, EFT, acupuncture or a good massage might be helpful to speed up the healing process, and reduce the discomfort.
• And, on rare occasions, a reduction of the dosage or temporary cessation may be required.