Why did the 1918 outbreak of the spanish flu cause so many deaths? the virus had combined with an even deadlier avian flu virus. the united nations could not distribute vaccines fast enough. the influenza virus was a strain to which people had no immunity. the world health organization had not yet been created.

The influenza virus was a strain to which people had no immunity.

Explanation:

The 1918 influenza pandemic (known as Spanish flu) was a pandemic involving the H1N1 influenza virus. This was one of the deadliest natural disasters in human history, resulting in the death of three to five percent of the world's population.

The pandemic had a higher than expected mortality rate for young adults, and there is no consensus as to why this was the case. Some theories establish that the strain was particularly deadly, and that people had no immunity to it. Some others argue that due to the war, malnourishment, overcrowding and poor hygiene were common, leading to worse outcomes for the sick.

One of the most important reasons was for the ban imposed on Japan trade functions by President Roosevelt which gave a great set back to the Japanese economy and their trade and military was affected. 
The Japanese were on their way to emerge as the strongest naval power but America was standing in their way, they had to do something about America's great destroyer ships and naval personnel. Also another reason that Japan attacked America is because the US wanted Japan to withdraw from the north-Indo China. All of these reasons combined led to Japan attacking America's navy in Pearl Harbor.