The primary stage of syphilis is usually marked by the appearance of a single sore, but you may get multiple sores. The sore begins as a pimple, but it quickly changes into a firm, round, small, and painless ulcer. If it is inside the vagina or anus, it is easy to miss this stage. The sore generally lasts 3 to 6 weeks, and it heals with or without treatment. At this stage, people are highly infectious.