When the great influenza pandemic of 1918-1919 struck, an estimated 675,000 Americans died. That was ten times the number of Americans who died in World War I, which was then coming to an end in Europe. Sometimes called the Spanish flu, this influenza, which initially seemed as benign as a common cold, would ultimately take between 20 million and 40 million lives worldwide, killing more people than the bubonic plague had killed from 1347 to 1351. The U.S. Centers for Disease Control and Prevention (CDC) estimates that nearly one-third of the world's population was infected with the influenza virus, which was surprisingly virulent among the young and fit. Today, live samples of the 1918 virus and evolving RNA science give researchers a good picture of the structure of this flu strain, yet both the origin and cause of this particular catastrophic plague episode remain a mystery. The CDC's interpretation of the data is chilling; until the root causes of the 1918-1919 pandemic are understood thoroughly, "analogous conditions could lead to an equally devastating pandemic."

Which of the following CANNOT be inferred from the passage?

1)Scientists were somehow able to recover a live sample of the 1918 virus.
2)International borders and even oceans were inadequate barriers to the spread of the 1918 flu.
3)World War I led to the 1918–1919 flu pandemic.
4)Scientists are making an effort to unravel the causes of the 1918 flu epidemic.
5)Flu viruses normally strike less commonly among the young and fit.