Okay, no problem.
I'm going to talk in French to be very specific.
A flu pandemic clearly involves an infinitely transmissible disease, by definition. A virus that would cause a pandemic would be very virulent and very easy to transmit. Currently, when such cases first appear, we can try to quarantine the people involved and stop the disease from spreading. Nevertheless, sooner or later, it is very likely that despite all our attempts, the disease will keep on spreading.
Let me come back to your example. We are riding on a bus and a passenger is incubating the pandemic flu. He might not yet be extremely ill, but he can be contagious. This is how pandemic flu behaves. Patients may not show clear symptoms and be contagious nonetheless. In such cases, we can do something, of course, but when the disease is highly contagious, it is very difficult.
It was discovered that SARS was not very contagious unless you got very close to a patient. As a matter of fact, if we look at the places where the disease spread the most, we realize that those places had many problems with hygiene, even in hospitals. When we look at the results of the attempts to monitor fever in order to detect SARS, we realize that this procedure was ineffective in airports, trains and buses.