Ebola is the latest in a long list of infectious disease outbreaks that have plagued humankind. The worst become pandemics, sweeping across continents and killing as many as tens of millions of people.
Today, nations are struggling to get Ebola under control. This week saw New York City's first case, while the World Health Organization declared Nigeria and Senegal free of Ebola. (See "From Senegal and Nigeria, 4 Lessons on How to Stop Ebola.")
How much higher this epidemic's death toll might climb remains to be seen. In late September, the U.S. Centers for Disease Control and Prevention estimated that in a worst-case scenario, the total number of cases (not deaths) could reach 1.4 million by late January.
1. Plague of Justinian (541-542):This bubonic plague outbreak spread throughout the Byzantine Empire, centered on the Mediterranean region. It halted the reconquest of lands once part of the Roman Empire. Estimates vary widely for the plague's death toll. The CDC estimates that it "eventually killed over 100 million people."
2. Cholera: Cholera pandemics started in India in the 1800s. The seventh outbreak (still ongoing) started in Indonesia in 1961 and spread across Asia, the Middle East, and Africa.
3. Flu: Influenza pandemics have flared up several times and likely will again, as flu viruses mutate into new strains. The Spanish flu of 1918 was particularly deadly, with worldwide death totals ranging from 20 to 50 million people.
4. Typhus epidemic of 1847: Massive numbers died in Ireland. Others, fleeing disease and the famine of 1846, died in "coffin ships" and upon arrival as immigrants to U.S. and Canadian ports.
5. Great Plague of London (1665-1666): The last in a long series of London bubonic plagues that started in 1499, this epidemic killed about 20 percent of Londoners. It started to subside in early 1666 and finally ended after the Great Fire of London later that year.
Updated Oct. 29 with additional information about Justinian Plague and Spanish flu estimates.