In the early part of the sixteenth century, a new disease spread like wildfire in Europe. This was syphilis, a sexually-transmitted disease. It was especially virulent, and was sometimes called the Great Pox, a distinction from smallpox.
Europeans blamed neighboring peoples as the source of the disease: the French disease, the English disease, etc.
And, of course, condoms were referred to later as 'French letters.'
If condoms were not used, the offspring were called 'bastards.'