Disease is understood to be something that can rise when its flow of Chi is somehow disrupted in one or more meridians, or one or more areas of the body. Blockage of the Chi can result in pain, as well as dysfunction of organs or other tissues, and can have a broad impact on one's entire system. This is simply a way of describing every day, observable events. When one sprains an ankle, they may have pain alongside of the leg, not just in the ankle, but all the way to the hip, even. This is obstruction of the Chi from trauma.
Acupuncture can be very effective in the treatment of low back pain.
In fact, low back pain is one of the conditions that interested me, and led me to the study of acupuncture in the first place. Low back pain can be caused by many things, so it wouldn't be right for me to speak too broadly about acupuncture's effect in low back pain without being a little bit specific. The simplest perspective might be to say that acupuncture can be helpful by reducing swelling and inflammation, and by doing so, relieving pain. This is one of the explanations I often use at work. Much of my practice is in a large, multi-disciplinary clinic and hospital, and I'm working with physicians who need to understand quickly how to make a referral.