"The outbreak began in 1918, during the final months of World War I, and historians now believe that the conflict may have been partly responsible for spreading the virus. On the Western Front, soldiers living in cramped, dirty and damp conditions became ill. This was a direct result of weakened immune systems from malnourishment. Their illnesses, which were known as "la grippe," were infectious, and spread among the ranks. Within around three days of becoming ill, many soldiers would start to feel better, but not all would make it. During the summer of 1918, as troops began to return home on leave, they brought with them the undetected virus that had made them ill. The virus spread across cities, towns and villages in the soldiers' home countries. Many of those infected, both soldiers and civilians, did not recover rapidly. The virus was hardest on young adults between the ages of 20 and 30 who had previously been healthy."