Re: Diseases
Shak first it is not known how many people had been affected before. The information spread because of a group of people got affected.
Secondly, the viruses are ever changing organisms. It takes a long time for genetic compositon of these agents or hosts, to result in what we see at some pont of time.
Who knows some good viruses in future become lethal by some mutations. Or vice versa.
There are other diseases or condtions which are presented this way. It is called Acquired deficiency (AIDS) to dfferentiate it from some genetic immunoeficiency condtions.