How did the Rifleman's wife die?

Answer 1

She died after Mark was born. She died from an epidemic sickness in the midwest, before the Rifleman and Mark moved to Northfork.

Answer 2

She died after her son Mark was born. She contracted an epidemic sickness and died in the midwest. After her death the Rifleman moved to Northfork.

Answer 3

She died in the Midwest from an epidemic sickness.