Heres the google ngram for the word syphilis. I was wondering what these three peaks could be.
Peak 1 - around 1870s - Repeal of the Contagious diseases act
Peak 2 - around 1910s - Discovery of Treponema and Salvarsan
Peak 3 - around 1940s - World War and Penicillin
This is just a speculation and I don't know for sure. What are the correct explanations?
I don't know if anyone can give the "correct rationale" for the peaks, but we can make some educated guesses.
I agree with your explanations for the rise and peaks in the 1870s/80s and 1940s. The Contagious Diseases Acts were quite controversial since their passage, and the press for their repeal gained quite a lot of publicity. Additionally, penicillin was a major breakthrough in treating a number of different diseases. I'd also argue that Nelson and Mayer's creation of the Treponema pallidum immobilization test (TPI) in 1949 may have played a role in the 1940s peak (see Tampa et al. (2014)).
I also agree that the early developments regarding Treponema pallidum partially caused the early peak in the 1910s, first with its discovery in patients with syphilis and then the link between it and neurosyphilis, as put forth by Noguchi in 1913. There were other major breakthroughs around this time. You mentioned that Salvarsan and Neosalvarsan were major treatment options, thanks to Ehrlich. However, its application to war may have been what resulted in the peak. LW Harrison wrote quite a lot on this. Furthermore, developments in the use of bismuth (see here) in 1916 and then 1921 made it a second option, replacing arsenic. Without World War I, Ehrlich's discovery might not have gained quite as much attention. Also, the Wasserman test, developed in 1906 contributed to the diagnosis of syphilis.
There's one more amusing piece of information that may have led to a data bias in the late 19th and early 20th centuries. Syphilis cases rose in the United States between 1900 and 1920, and it was around this time that newspapers began using the term "syphilis":
Indeed, discussion of the disease only took place in specialized books and in medical journals, not in publications that most people would have read. Viewed as a subject beyond the "boundaries of decency," syphilis was thought to be a disorder that affected only the immoral. In fact, the American press, yielding to the desires of the common people, was so unwilling to deal with the matter that the Reader's Guide to Periodical Literature did not include "syphilis" as a heading until 1907, and the New York Times Index avoided the term until late 1917.
This general trend should also account for a strong increase and then peak in the late 1910s/early 1920s.
I think this question is mainly about word usage. The English name for this disease is “pox”. You can find it dozens of times in Shakespeare, Swift etc. “Syphilis” started off as the personal name of a fictitious person (supposedly the first victim of the disease), the protagonist of a Latin poem from the 16th century. Later it became the pseudo-Latin medical designation for the disease. The surge in the usage of “syphilis” in English between 1850 and 1880 is presumably the result of Victorian prudery, which replaced “pox” by a more neutral sounding Latinate term. After 1880 we see a second wave of prudery, where “syphilis” itself becomes a dirty word and gets replaced by “infectious diseases” and the like. It comes back into favour at the time of the two world wars, when the dangers posed to British and American soldiers by women in the war zones is a frequent topic of the press.
