The war between Russia and Sweden continued after the disaster of Poltava in 1709, though the shattered Swedish continental army could provide very little help. Russia captured Viborg  in 1710 and successfully held it against Swedish attempts to retake the town in 1711. In 1712 the first Russian campaign to capture Finland began under the  command of General Admiral Fyodor Apraksin. Apraksin gathered an army of 15,000 men at Vyborg and started the operation in late August. Swedish General Georg Henrik Lybecker chose not to face the Russians with his 7,500 men in the prepared positions close to Vyborg and instead withdrew west of Kymijoki river using scorched earth tactics. Apraksin's forces reached the river but chose not to cross it and instead withdrew back to Vyborg, likely due to problems in supply. Swedish efforts to maintain their defences were greatly hampered by the drain of manpower by the continental army and various garrisons around the Baltic Sea as well as by the plague outbreak that struck Finland and Sweden]] between 1710-1713, which devastated the land killing, amongst others, over half of the population of Helsingfors .
Question: What killed the Helsingfors?
Answer: plague outbreak