The other day when I was flying, a passenger was denied boarding because they were ill.
I don't know all the specifics of the passenger's symptoms, as it happened on the ground, all I know is that there were lots of people grumbling that an airline shouldn't be able to decide someone is too sick to travel. After all, the passenger just wanted to get home.
Flying a bunch of people around in a long metal tube involves some issues of public health.
And if you don't think so, consider that passengers infected with the mumps caused the disease to spread to epidemic proportions in seven miswestern states. People were exposed in the airport as well as on the plane.
To date, over 600 suspected cases of mumps have been reported in Iowa , Nebraska, Kansas, Illinois, Missouri, Wisconsin, and Minnesota.
The incubation period for months is two to three weeks, so it's entirely possible we will see more cases of the disease appearing in the next week.
Remember -- if you're sick, do yourself and everyone, else a favor -- don't fly!