Final answer:
The sentence indicates that leprosy and syphilis are two separate diseases; leprosy is not sexually transmitted, whereas syphilis is a sexually transmitted infection caused by a specific bacterium.
Step-by-step explanation:
The main idea of the sentence "Leprosy was infectious but had nothing to do with syphilis" is (A) Leprosy and syphilis are two different diseases. While both leprosy and syphilis are infectious diseases, they are caused by different pathogens and have different modes of transmission. Leprosy is primarily transmitted through droplets from the nose and mouth during close and frequent contacts with untreated cases, whereas syphilis is primarily a sexually transmitted infection caused by the bacterium Treponema pallidum, which requires close physical contact for transmission.