Ordinarily, birds do not even get sick when they have the flu. In Hong Kong, people like to buy chickens at markets where they are kept live, in cages, and killed on the spot. It's the ultimate in freshness, but it was spreading a deadly flu. So, the Hong Kong government ordered that every bird in Hong Kong be killed. That meant that 1.2 million chickens, geese, and ducks were slaughtered.
Scientists still shudder over that flu.
But they worried that if a person was infected with the chicken flu and, simultaneously, with a human flu, the two flus could mingle in the person's lungs and what emerged could be a deadly flu that could spread easily from person to person. They worried that it might be 1918 all over again. The young people who were so sick had the typical flu symptoms. But then the symptoms got much worse. The patients could not breathe, their lungs filled with fluid.