people died. During this pandemic, the United States saw its first outbreak: the San Francisco plague of 1900–1904. Today, sporadic cases of plague still occur in the western United States. The 1918–1920 Spanish flu infected half a billion people around the world, including on remote Pacific islands and in the Arctic—killing 20 to 100 million. Most influenza outbreaks disproportionately kill the very young and the very old, but the 1918 pandemic had an unusually high mortality rate for young adults. It killed more people in 25 weeks than AIDS did in its first 25 years. Mass troop movements and close quarters during World WarI caused it to spread and mutate faster, and the susceptibility of soldiers to the flu may have been increased by stress, malnourishment and chemical attacks. Improved transportation systems made it easier for soldiers, sailors and civilian travelers to spread the disease. Encounters between European explorers and populations in the rest of the world often introduced epidemics of extraordinary virulence. Disease killed part of the native population of the Canary Islands in the 16th century (Guanches). Half the native population of Hispaniola in 1518 was killed by smallpox. Smallpox also ravaged Mexico in the 1520s, killing 150,000 in Tenochtitlán alone, including the emperor, and in Peru in the 1530s, aiding the European conquerors. Measles killed a further two million native Mexicans in the 17th century. In 1618–1619, smallpox wiped out 90% of the Massachusetts Bay Native Americans. During the 1770s, smallpox killed at least 30% of the Pacific Northwest Native Americans. Smallpox epidemics in 1780–1782 and 1837–1838 brought devastation and drastic depopulation among the Plains Indians. Some believe the death of up to 95% of the Native American population of the New World was caused by Europeans introducing Old World diseases such as smallpox, measles and influenza. Over the centuries, Europeans had developed high degrees of herd immunity to these diseases, while the indigenous peoples had no such immunity. Smallpox, introduced by European settlers in 1789 to the Australian continent, devastated the Australian Aboriginal population, killing up to 50% of those infected with the disease during the first decades of colonisation. It also killed many New Zealand Māori. In 1848–49, as many as 40,000 out of 150,000 Hawaiians are estimated to have died of measles, whooping cough and influenza. Introduced diseases, notably smallpox, nearly wiped out the native population of Easter Island. Measles killed more than 40,000