In the realm of infectious diseases, a pandemic is the worst case scenario. When an epidemic spreads beyond a country's borders, that's when the disease officially becomes a pandemic.
Communicable diseases existed during humankind's hunter-gatherer days, but the shift to agrarian life 10,000 years ago created communities that made epidemics more possible. Malaria, tuberculosis, leprosy, influenza, smallpox and others first appeared during this period.
The more civilised humans became, building cities and forging trade routes to connect with other cities, and waging wars with them, the more likely pandemics became. See a timeline below of pandemics that, in ravaging human populations, changed history.430 B.C.: Athens
The earliest recorded pandemic happened during the Peloponnesian War. After the disease passed through Libya, Ethiopia and Egypt, it crossed the Athenian walls as the Spartans laid siege. As much as two-thirds of the population died.
The symptoms included fever, thirst, bloody throat and tongue, red skin and lesions. The disease, suspected to have been typhoid fever, weakened the Athenians significantly and was a significant factor in their defeat by the Spartans.165 A.D.: Antonine Plague
The Antonine plague was possibly an early appearance of smallpox that began with the Huns. The Huns then infected the Germans, who passed it to the Romans and then returning troops spread it throughout the Roman empire. Symptoms included fever, sore throat, diarrhoea and, if the patient lived long enough, pus-filled sores. This plague continued until about 180 A.D., claiming Emperor Marcus Aurelius as one of its victims.
250 A.D.: Cyprian Plague
Named after the first known victim, the Christian bishop of Carthage, the Cyprian plague entailed diarrhoea, vomiting, throat ulcers, fever and gangrenous hands and feet.
City dwellers fled to the country to escape infection but instead spread the disease further. Possibly starting in Ethiopia, it passed through Northern Africa, into Rome, then onto Egypt and northward.
There were recurring outbreaks over the next three centuries. In 444 A.D., it hit Britain and obstructed defense efforts against the Picts and the Scots, causing the British to seek help from the Saxons, who would soon control the island.