Final answer:
Secondary syphilis is the most infectious stage of syphilis, characterized by the spread of the bacteria throughout the body and systemic symptoms.
Step-by-step explanation:
The most infectious stage of syphilis is secondary syphilis. During this stage, the bacteria spreads throughout the body, causing systemic symptoms such as skin rashes and sores on the mucous membranes. Secondary syphilis is highly contagious, and individuals are most likely to transmit the infection during this stage.