evidence: The Spanish flu killed quickly, and it killed in huge numbers. Other flu pandemics in modern times have been far less deadly. Why? [...] Many flu deaths are also caused by secondary, bacterial infections that take root in the weakened body, leading to pneumonia. Antibiotics like penicillin – discovered in 1928 – now allow doctors to reduce that risk, but in 1918 there was no such treatment. Nor did they have vaccines, which now help to protect those who are most at risk. "Our health care infrastructure and diagnostic and therapeutic tools are so much more advanced," says Jessica Belser, who works at the influenza division of the US Centers for Disease Control and Prevention. [...] "It’s well understood that improved sanitation and hygiene, associated with industrialisation and general reductions in poverty, have contributed significantly to overall reductions in infectious disease mortality in the 20th Century," agrees Kyra Grantz at the University of Florida. Analysing records from Chicago during the 1918 pandemic, she has shown that factors such as population density and unemployment directly influenced someone’s chance of catching the disease.