Transmission is usually only possible when the virus is outside the cell, meaning it's replicated, the virions are assembled, and the host cell has been lysed to release the complete virus. Also, you need a critical amount of the extracellular virus circulating in body fluids to successfully infect someone. So generally, symptoms appear when a certain amount of damage (aka cell/tissue lysis) has been done because 1) that's when the hemorrhage part of hemorrhagic fever really kicks in and 2) that's when there's enough foreign antigens for your immune system to detect them and respond.
So to sum it all up, when you get symptoms is when there's a "critical mass" of viral load for you to start infecting others.
