The virus spread like wildfire and killed its victims by causing fluid buildup in the lungs - pulmonary edema - or by compromising the immune system so bacterial pneumonia developed.
I had an ancestor in Fort Collins CO who went to work healthy, felt sick at noon, asked to leave early, then went home and died that very evening.
Now imagine a flu virus is on the loose. It has mutated so its effect on humans is severe to fatal. A person on vacation or business visits the locale, and contracts the virus. He flies home that day, infecting all who he breathes on or touches. He interacts with family members. They now have the virus. He goes to work the next day. He attributes his headache and lack of energy to jet lag. He gets through the day, and goes home feeling drained. He self-medicates with alcoholic beverages and so fails to realize he's getting sicker.
The next morning his wife finds him collapsed in the bathroom. Over his mumbled protests she calls the hospital. He's admitted and tests are done. The results have everyone scratching their heads. Someone thinks to ask where the man has traveled lately. A tropical disease specialist is called in. Various noxious diseases are ruled out. Then man is very ill indeed now. He has fluid in his lungs. At last someone compares him with flu victims from medical journals. He's rushed to an isolation ward and the Center for Disease Control is notified. All the while everyone is spreading the infection.
New flu victims start to trickle into hospitals in the area, and in some far-flung locales. The trickle becomes a flood in 2 days. The man who brought the flu home succumbs to pulmonary edema. He is the first victim. Will there be hundreds more?
Thousands?
Millions?
It sure would be nice if the government of China could maybe spend a little less on missiles and bombs and airplanes and nuclear weapons and maybe spend a little more on plumbing and sanitation and toilets and hygene classes for the masses. Just a thought. Might spare us all a major epidemic.