Bacteria and viruses are too small to be seen, in fact they have to be in millions before we can recognize them as a colony. We never realize the power of microbes until we are affected by them. We all can imagine someone coughing in your face, then realizing three days later that was the source of your cold. These were the situations we tried to avoid. Then many of us started to slowly realize that some colleagues and acquaintances over time had had problems with disease that were occupationally oriented.
All infections have an incubation period.
The most difficult concept for us to fathom is that microbes are truly invisible and that every time we have contact with microbes, we become infected. Now, becoming infected does not always lead to sickness or disease. After infection our immune system fights the invading microbes with varying degrees of success. This Incubation Period, between the time of infection and the onset of disease is different for all microbes. For Influenza, it may be a few days, but the frightening aspects of incubation periods are for Hepatitis and Tuberculosis.