The Spanish flu
The Spanish flu was an unusually difficult and deadly form of flu, which spread all over the world at the end of, and after the first world war. Its source is unknown. Most of the casualties were young adults, who otherwise were in good health. Normally in flu epidemics, children, old people and those with a decreased immune system are worst affected.Well done Emanuel ... this is a well written short article and I hope the corrections help.
The Spanish flu
The Spanish flu was an unusually difficult and deadly form of flu, which spread all over the world by/towards the end of and after the first world war. Its source is unknown. Most of the casualties were young adults, who otherwise were in good health. Normally in flu epidemics there are instead it is children, old adults and people with a weakened immune system who are the worst affected."old adults" is correct, but "the elderly" sounds better
humanity is correct, but I like the phrase "in the history of mankind".
difficult - you are looking for some other word here. I think "insidious" is a good word to descirbe such a disease.
the - I think there should be a "the"