The Inca governor at Tumbes sent his runners with all speed to the Inca Wayna Capac who was near Quito resting after battle. No doubt, too, he was weighing up the sinister news of the outbreak of pestilence in the heart of his empire. But he had only moved a short way south when the disease struck his camp. The incubation period of smallpox is only a few days and, in no time, it swept through the army. Many of his trusted generals died; and then the Inca himself caught it.

As Wayna Capac's health rapidly worsened, he was asked to name a successor to be ratified by his council of wise men. Sources disagreed on his choice: some said it was his 25-year-old son Atahuallpa and others said it was 21-year-old Huascar — Atahuallpa's younger brother by a different queen. Fate could not have played the Inca a worse card. The empire plunged into a bloody civil war.

Nueva Corónica y Buen Gobierno
(Letter to a King)

Although the title of Inca was hereditary, primogeniture was not a factor in the Inca royal succession. The dying Inca could choose the son he felt was best suited to succeed him. Atahuallpa and his younger brother, Huascar became entrenched in a bloody civil war for power after their father Wayna Capac succumbed to smallpox.