Paget's disease of bone is characterized by the progressive and extensive 
replacement, in one or several bones, of normal bone tissue by a bone tissue of 
rough and irregular structure, the excessive and disorderly renewal of which 
gradually produces hyperdensity and hypertrophy of the bones involved. The 
condition results from the action of abnormal and overactive osteoclasts 
containing virus-like intranuclear and intracytoplasmic inclusions. The disease 
profoundly alters the physiology of bones in the region affected. The 
viral-looking inclusions that enter the osteoclasts seem to induce a loss of 
control of bone renewal and remodelling, as shown by histology and radiology. 
The resorption-formation mechanism persists but it is unbalanced, facilitating 
bone resorption during two to five years, then bone formation. In that way, 
subjects with Paget's disease, who have reached the age of physiological 
osteopenia, show an often important increase in bone density and mass. Paget's 
disease of bone is a disease grafted on the skeleton: it is partly dependent on 
the skeletal status of the host. The activity of Paget's disease is evaluated by 
measuring the ratio of plasma alkaline phosphatase levels to the volumes of 
normal and pagetic bones; the author suggests a mathematical formula for 
calculating this ratio.