When the WHO declares a pandemic it is not usually due to the plague resurfacing with bodies dropping by the thousand. That is not a pandemic, that is the result of ignoring a disease and letting it get out of control.
Infection is not necessary. With parasites infection tends to be a predictable set of symptoms but viruses like the flu are by their very nature able to steal genes from the host cell and thus sneak into a back door, latch on and start feeding upon a host body. That is not particularly scientific but the problem here is that genetic information is a collection of characteristics with a rapid form of direct evolution binding in more and more of our genetic information as it infects different hosts.
The danger may not be the initial vector. If a pandemic is left to fester, the virus may as a result become far more virulent as it gathers more human information it makes us all more vulnerable to being its prey.
The control of SARS is oft quoted in media as a “reason we should not be worried” but considerable vigilance may have prevented mass infection with the deaths of millions and we can be thankful that it is not so much of a problem.
We have belief and trust in laboratory analysis as it declares a victim has “H1N1” without really understanding what that means. Does not stop the media spouting that expertise like facts that can be easily digested and creating panic. Understanding how viruses work is not casual knowledge.