Chiropractic was was invented by Canadian Daniel D Palmer 1895 when after adjusting the spine of a deaf man, allegedly restored his hearing. Palmer - an anti-vaccine proponent and alt-med practitioner - believed that the body had its own healing power, transmitted via the nerves and that illness was caused by spinal misalignment (called subluxations) blocking the nerves carrying this healing power. Consequently manipulation of the spine could free these subluxations and result in healing.
Real subluxations (partial dislocations) show up on x-ray. Chiropractic “subluxations” do not and therefore modern chiropractors had to change their definitions. Over the years they have tried to move into mainstream medical treatments. Chiropractors are divided into straights and mixers. The former concentrate on chiropractic adjustments whereas the mixers add in other methods ranging from massage to outright quackery like applied kinesiology. There is some evidence that some spinal manipulation therapy is good for short-term relief from back pain but it is no better than conventional physiotherapy. There is no evidence that it is any good as a treatment for any other condition and it may actually be dangerous; neck manipulation carries risk of stroke or paralysis.
In their book Trick or Treatment, Simon Singh and Edzard Ernst reviewed the evidence for chiropractic and concluded: “Chiropractors… might compete with physiotherapists in terms of treating some back problems, but all their other claims are beyond belief and can carry a range of significant risks.”