It's a common trope throughout humanity's patriarchal history: The otherness of woman is not only alien and unknowable but ultimately the stuff of monstrosity.
We see it in myth cycles, folklore, witchcraft persecution and various interpretations of human reproduction. Rather than equal (or, realistically, greater) examples of the species, women are seen as beings of bloody cycles, physical corruption and moral failing.
It should come as no surprise that such misogynistic garbage turns up in the history of syphilis. As explored in our podcast episodes on the topic, this highly-destructive disease plagued Europe unchecked for more than four centuries. While the disease spread through all levels of society, it saw a great deal of traffic through soldiers and prostitutes. This reality continued even in the advent of syphilis-slaying antibiotics in the 20th century, which is why we see so many posters warning male soldiers about the feminine danger of syphilis.
Granted, these were aimed at a male audience. They preach a very male-centric, disease-based anti-prostitution message. But just as centuries-old theories blamed syphilis on female monstrosity (the semen of multiple partners corrupts into syphilis in the prostitute's womb!), the images seethe with an undercurrent of sexism.
Fear her, they whisper. Fear the monstrous feminine.
Let's look at some more examples...
And while I'm at it, here's a close-up from the lead image:
Now let's look at a syphilis posters which at least had the decency to envision syphilis as a straight-up monster rather than an extension of female corruption...
This one's interesting in that discusses syphilis as an ancient illness that even predates humanity. As discussed in our podcast episode "Syphilis Through the Ages," this probably isn't in the case. Also, you can't catch it from a dinosaur FWIW.
And finally, here's a recent poster from Canada's CATIE (Centro Agronómico Tropical de Investigación) that delivers an anti-syphilis message at gay and bisexual men. It borrows imagery from "Creature From the Black Lagoon" in order to drive home the monstrous nature of the illness.