Abraham Lincoln vs. Zombies

After his mother Nancy Lincoln (Rhianna Van Helton), falls victim to an illness that requires her to be tied to her bed, vicious and cannibalistic, 10-year-old Abe Lincoln (Brennen Harper) sees his father Thomas Lincoln (Kent Igleheart) commit suicide at her bedside. Taking up a scythe, the distraught young Abe tells his mother that he loves her before beheading her. He then joins others in his community in containing a local zombie outbreak.
When an adult Abraham Lincoln (Bill Oberst Jr.) has become President of a fracturing United States, he is apprised of rumors concerning a prominent Confederate stronghold. He is told that a regiment of 30 men had gone to Confederate Fort Pulaski to seize it from the Confederates and only one man returned barely alive. When questioning the survivor, Lincoln discovers the soldier has an illness that seems to bring corpses back to life. He then personally leads a team of the newly established secret service to accompany him in investigating the fort.
They get to the fort and are attacked by Confederate survivors led by General Stonewall Jackson (Don McGraw) as well as by several of the infected people. Abe kills one of his men who had been bitten by an infected and explains to the others in his party that if they are bitten or scratched by an infected, or if an infected's blood makes contact with their mouths or eyes, that they too will become infected and, within twenty-four hours, would no longer be considered "human". More are lost when attempting to investigate the surrounding neighborhood, and Abe encounters an old flame turned prostitute Mary Owens (Baby Norman) who is sheltering her help, including a young Theodore Roosevelt (Canon Kuipers), daughter Sophia (Hannah Bryan) and prostitute Annika (Anna Fricks) to protect them from the invasion.
The group makes their way back to the fort where Jackson refuses to kill the infected, believing them to only be sick and in need of care. He claims Lincoln's actions are only against the members of the confederacy. Based on an old Bantu word he'd learned from his mother, Agent Brown (Jason Hughley) names the infected persons "zombies". Pat Garrett (Christopher Marrone) volunteers to help Abe, and guides them to a nearby plantation that has farming implements and other weapons they can use to combat the threat.
When returning to the fort, Mary is splashed in the face with zombie blood and soon falls ill. Meanwhile, agent John Wilkinson (Jason Vail) protests the killing and asks to remain behind as the rest of the group heads into the township to kill off the infected. Being greatly outnumbered, only Abe, Theodore, and Sophia return when the others are slaughtered. Wilkinson, who is revealed to be a spy, plots to kill Lincoln while he's alone but changes his mind when he catches Abe praying, as in his mind prayer would ensure Lincoln's soul going to heaven. After being convinced by Pat Garrett that Lincoln is right and escape is unlikely, Stonewall shows Abe a secret cache of gunpowder. They then decide to use the explosive to blow up the fort after trapping and containing the zombies inside. When the fuse goes out, Stonewall ventures down alone to re-light it, but is over-run and killed by the zombies just after doing so. Abe and Brown escape just in time and the entire place goes up. Mary accepts her fate and goes off with Lincoln to die, much to Sophia's heartbreak.
Eighteen months later, Abe goes to visit Mary who had been in the care of a doctor investigating the illness in vain hope of finding a cure. As Abe cleans wounds caused by her restraining shackles, Mary grabs his hand scratches his skin infecting him, much to his horror, forcing him to kill her. Knowing he is himself uncurable, Abe requests that a message be sent to John Wilkinson, as earlier he'd discovered that Wilkinson was actually John Wilkes Booth who had a plot to kidnap Lincoln in response to the end of the war and the Union the victor. The message gives Booth the information to know exactly where Lincoln would be the following night as Abe and his wife Mary go to the theater, thus leading to his assassination (purposely arranged by Lincoln to prevent another outbreak) at Booth's hands.

Question: How does Abe know Mary?
Answer: She is an old flame