The Phenomena website is running an article with good information about measles. It clears up something that was confusing me, which is exactly how dangerous this disease really is. An ordinary case of measles comes in through the lungs, attacks immune cells, circulates for a while, and ultimately moves back into the respiratory system, where it can be coughed back out in order to find a new host. There are a couple of characteristic ways for measles to get out of hand. One is that, on rare occasions, it spreads into the nervous system, with horrific results. Another is that it severely depresses the immune system for several weeks, leaving its sufferers vulnerable to dangerous bouts of opportunistic pneumonia. In a wealthy society with good medical care, this translates into one to three deaths per thousand. In a grisly refugee camp, the death rate can be 25%.
Measles, an airborne virus, is fantastically contagious. Something like 90% of non-immune people in a room with an infectious measles patient can expect to contract the disease.
This Hoover article runs through some of the legal history of the police power in the field of epidemiology and public health. It's an old controversy.