A few years ago a very simple test was discovered to tell whether a
person could or could not catch diphtheria. This is known as the Schick
test. It consists in injecting a few drops of a prepared diphtheria
toxin into the skin and then watching whether a characteristic red spot
appears where the injection was made. If such a spot does not appear
within two or three days it shows that the person can not catch
diphtheria.