>Laboratory studies on the reconstructed 1918 virus began in August 2005. A report of this work, “Characterization of the Reconstructed 1918 Spanish Influenza P

>Laboratory studies on the reconstructed 1918 virus began in August 2005. A report of this work, “Characterization of the Reconstructed 1918 Spanish Influenza Pandemic Virus”, was published in the October 7, 2005, issue of Science.14 To evaluate the 1918 virus’ pathogenicity (i.e., the ability of the virus to cause disease and harm a host), animal studies involving mice were conducted. The mice were infected with the 1918 virus, and measures of morbidity (i.e., weight loss, virus replication, and 50% lethal dose titers) were collected and documented. For comparison, other mice were infected with different influenza viruses that were designed via reverse genetics to have varying combinations of genes from the 1918 virus and contemporary human seasonal influenza A(H1N1) viruses. These viruses are called “recombinant viruses.” >The fully reconstructed 1918 virus was striking in terms of its ability to quickly replicate, i.e., make copies of itself and spread infection in the lungs of infected mice. For example, four days after infection, the amount of 1918 virus found in the lung tissue of infected mice was 39,000 times higher than that produced by one of the comparison recombinant flu viruses. https://www.cdc.gov/flu/pandemic-resources/reconstruction-1918-virus.html https://www.cdc.gov/flu/pandemic-resources/reconstruction-19... The referenced page goes on to detail how various parts of the 1918 virus contributed to virulence. I've seen articles that said that the reason 20-40 year old patient had more severe disease is that their immune systems were stronger, and that the lung damage is actually caused by the immune system combating the virus. The 1918 virus started out earlier during WW I. During that time, large groups of men were being transported internationally by troop ships. While trains and ships are slower than airplanes a journey from a camp in the US to one in France probably took under two weeks.