How long are you actually contagious after the fever breaks?

It depends on the infection, and the fever breaking is not the same as being non-infectious. For influenza, most adults are infectious from about a day before symptoms until roughly five to seven days after they start, with children often longer. For COVID, the usual guidance is isolation until symptoms improve and you have been fever-free for 24 hours without paracetamol. For a common cold, the first three days are the most infectious. The practical rule: feeling better is not the finish line, being fever-free without medication for a full day is closer.