The Aztecs were affected by several diseases that caused significant damage to their population. The most devastating epidemic was the cocoliztli, which occurred between 1545 and 1550 and killed an estimated 5 to 15 million people, or 80% of the population. The exact cause of the epidemic has been debated for centuries, but recent studies suggest that a lethal form of salmonella, Salmonella enterica, subspecies enterica serovar Paratyphi C, was responsible for the outbreak. The disease caused high fevers, headaches, and bleeding from the eyes, mouth, and nose, and death usually followed in three or four days. The epidemic was the second of three that were most devastating and led to the largest number of human losses. Smallpox was another disease that affected the Aztecs, killing an estimated 5 to 8 million people shortly after the Spanish arrived in Mexico in 1519. The disease killed many of its victims outright, particularly infants and young children, and incapacitated many other adults. The Spanish also brought other infectious diseases, such as measles, mumps, and influenza, which affected the native population.