For the flu (Covid is different - all viruses are generally speaking) Symptoms of the flu can manifest anywhere from 1 hour to 4 days after initial exposure, however most symptoms are presented around two days after initial exposure. You can be contagious anywhere from one day before symptoms (so that would probably assume as quickly as you have a transmittable viral load) and up to 7 days after symptoms resolve. “Immediate” would be like you touched flu, touched your nose to infect yourself and then touched an object that you squish the flu on to and then someone else then touches that item - immediately transmitting the virus - but if we are talking from moment it touches a cell that it infects to the viral load being large enough for say - a cough or bodily fluid to infect someone else - that would be highly dependent on the viral load you received, the rate at which this particular strain multiplied and how quickly your immune system kills them. I always assume immediately after exposure- I let people know I have it and if they around anyone vulnerable, similar to my Covid protocols. And I wear a mask for about a week after. For testing, once symptoms are present, you can test.