BIOGRAPHY - page 1
Mary Walker is a mutant suffering from mental illness. She was abused repeatedly as a young child, apparently by her father and in a sexual manner, a trauma with which her mind struggled to cope. [Marvel Comics Presents (1st series) #129] One account indicated Mary burned her 
house down as a child, possibly killing her parents in the process, although these memories were reportedly altered from what really happened at the time. [Typhoid Fever: X-Men #1] From an early age, she demonstrated signs of Dissociative Identity Disorder, an affliction previously identified as Multiple Personality Disorder. As she aged, two distinct personalities became apparent, each with their own mental and physical qualities. "Mary" was a pleasant if submissive child, exhibiting weakness and frailty that often manifested akin to epilepsy. Her other personality, however, earned the pseudonym "Typhoid" by being utterly poison: aggressive, malicious and manipulative. Although Typhoid showed no signs of Mary's epilepsy, she did experience a constant fever, keeping her body hot and her skin flushed red.
In addition to these strange but ultimately explainable symptoms, Mary and Typhoid also demonstrated paranormal attributes. Typhoid developed psychic powers thanks to her mutation, exhibiting forms of telepathy, telekinesis and pyrokinesis as she grew older. Mary seemingly could not access these abilities and blocked any knowledge of her powers or the existence of her second personality from her conscious mind. Mary believed she suffered spells or blackouts, but nothing more. Another strange aspect of their condition was a change in Mary's bio-readouts when she became Typhoid. Heart rate, EEG activity, brain wave patterns, even her scent changed with her persona. Mary was institutionalized since early childhood, but she (or rather Typhoid) eventually escaped as they approached adulthood. [Daredevil (1st series) #254]
[Note: Mary occasionally told the story of growing up poor with her blind father. He forced her to live in the dark with him, refusing to pay for electricity since he couldn't see the lights. This seems to contradict the idea of Mary being institutionalized from a young age. Since “Mary” told the story of her blind father in situations where Typhoid was manipulating her to get to others, it's possible this was only a story that Typhoid subliminally convinced Mary to tell from time to time.]
At nineteen years old, Mary was living at a brothel in New York City, working as a prostitute by the name "Lyla." A mob triggerman and regular customer named Angelo came in to lie low, but was 
pursued by a man in a mask. Angelo was one of the goons who killed Battlin' Jack Murdock for Mister Fixer and his pursuer was a disguised Matt Murdock. Lyla and the girls thought they were being raided and grabbed at Murdock to protect their meal ticket. Distracted by the smells and chaos of the brothel, Murdock panicked and lashed out, accidentally striking Lyla so that she went out the window, seemingly falling to her death. [Daredevil: the Man Without Fear #2] This was one of the turning points in Mary's life that led to the formation of her adult Typhoid persona, as she swore never to let a man hurt her again. [Deadpool / Daredevil Annual '97]
[Note: Lyla was for sure dead in the original story. In Daredevil (1st series) #350, Murdock even followed up with Lyla's friend Donna about her death and how he regretted killing the woman. The Deadpool / Daredevil retcon was only later established to equate Lyla and Typhoid, presumably to keep Matt Murdock from being a murderer.]
After a year on her own, Mary established herself as a stage actress. She appeared talented and well-received by her audiences, while many of her coaches, directors and fellow actors were incredibly smitten and on the verge of falling in love with her. This would be an ongoing factor in Mary's character, as men would quickly become attached to her and willing to support and protect her. This was presumably a subconscious application of her telepathic suggestion powers. Typhoid eventually resurfaced, however, and tore down Mary's success, leaving her acting career in the dust.
Years passed and, after spending some time in Chicago, Typhoid Mary arrived back in New York City. Typhoid stylized herself as a self-interested vigilante, killing drug dealers, racketeers and illegal gambling dens in order to help herself to the profits. She crossed paths with a mugger named Rip and quickly seduced him into becoming her murderous sidekick. Her efforts caught the attention of the Kingpin and his Arranger. The latter was concerned about Mary interrupting the cash flow of their businesses, but the Kingpin saw an opportunity in disguise.
Wilson Fisk was deeply immersed in a personal feud with his long-time arch-nemesis Matt Murdock, now the vigilante known as Daredevil. Months earlier, Kingpin had discovered 
Murdock's secret identity and attempted to break his opponent. He ruined Murdock's financial credit, orchestrated the removal of his license to practice law, alienated him from his colleagues and even blew up his brownstone. Despite all this, Murdock recovered and thrived. He reunited with his old girlfriend Karen Page and began working as a "ghost lawyer" for a legal aid clinic in Hell's Kitchen. The Devil bent, but he did not break.
Kingpin saw in Typhoid Mary a duality that perfectly mirrored Murdock and Daredevil. He sent word that Typhoid was to give him an audience. A curious Typhoid got close enough to put a blade to Fisk's throat in his own office. Impressed but not intimidated, Kingpin shrugged off Typhoid's blade and smothered one of her pyrokinetic fires with his bare hand. Fisk offered Typhoid one million dollars to break Murdock for him. They conspired to have him fall in love with Typhoid as Mary, coax him into betraying his love for Karen Page, then reveal Typhoid's true nature to shatter his heart.
At this point in their life, Typhoid was dominant enough to significantly influence Mary's behavior, even when Typhoid was the dormant personality. She could exert her powers of telepathic suggestion through Mary without Mary even being conscious it was happening. In this manner, she insinuated herself into a case on which Murdock was currently working. Kelco Industrial was allowing sewage to run off into a local river. These chemicals were responsible for blinding a boy named Tyrone, who had been playing in the river. Murdock was suing Kelso (secretly owned by Fisk) for damages. Mary presented herself as a social worker for the blind, an aid to help Tyrone rehabilitate himself and learn to function without sight. Typhoid subliminally drew Murdock's attention to Mary, bringing them together to bond over Tyrone's plight.
Mary's influence quickly overcame Matt's inhibitions and their time together became less about Tyrone and more about each other. Murdock was drawn to Mary due to her kindness, her vulnerability and her secret psychic seduction powers. A lingering touch soon turned into an embrace, which became more. Murdock believed he was in love with Mary so long as they were together, but he was wracked with guilt over what he was doing to Karen. More than once, he prepared himself to break things off with Mary, only to lose his resolve when they reconnected in person.
Meanwhile, Typhoid got in Daredevil's face on Kingpin's behalf. The changes to her bio-rhythms meant even Daredevil's heightened senses could not recognize Mary and Typhoid as the same woman. Typhoid assaulted Daredevil with her mania and aggressive sexuality while also trying to influence the jurors on the Kelco case for Kingpin. The goal was to make Daredevil hate Typhoid as much as he loved Mary. While working for Fisk, Typhoid also applied her powers to him, making him jealous of Rip and slowly seducing her employer as well.
Although she was having a good time all around, Typhoid discovered a concerning limit to her antics. While provoking Daredevil in a fight over a Kelco juror, Daredevil got under her skin by rejecting her, telling Typhoid how much she disgusted him. When the spiteful Typhoid impulsively tried to drown him for that, Mary surfaced against Typhoid's wishes, protecting Murdock and preventing his death. [Daredevil (1st series) #254-256]
While Murdock was just a job to toy with for Typhoid, Mary developed real feelings for Matt during their time together. Mary and Typhoid began to struggle for control, as Mary became increasingly aware of her double life. She even inadvertently responded to a meeting with the Kingpin instead 
of Typhoid. Typhoid convinced Fisk it wasn't a problem. She had gained significant control over Kingpin and the two of them were now openly sexual with one another, each believing the one was submitting to the other's dominance. [Daredevil (1st series) #257]
Days passed, however, and Typhoid soon could no longer convince Fisk she was just “savoring” the moment before breaking Daredevil emotionally. Her psychic seduction had made Kingpin jealous of the time Mary spent with Murdock and he wanted Murdock wrecked as soon as possible in order to have Typhoid to himself. Typhoid was privately forced to admit that Mary was growing too strong. She was preventing Typhoid from harming Daredevil directly and Typhoid feared Mary could even become dominant in their shared mind. Jealous of the love her other half was feeling, Typhoid decided to ditch the original plan and have Daredevil killed instead of just emotionally destroyed, removing him as a catalyst for Mary's growing mental strength. Unable to act directly thanks to Mary, Typhoid recruited Daredevil's enemies Bullet, Ammo, Bushwacker and the Wildboyz to run him through a gauntlet, weakening the red man until the final blow could be delivered by Typhoid with minimal effort. [Daredevil (1st series) #259]
Sure enough, Daredevil was beaten within an inch of his life by the villains set against him. In the end, Typhoid took over and went the last step, tossing the injured hero over a bridge with minimal effort. Even this small act weighed on her: it was Typhoid who threw him over, but Mary who shed a tear. When Kingpin was angered that Typhoid killed Murdock instead of merely torturing him, she shrugged it off and rightly pointed out Kingpin was too in love with her to really do anything about her insubordination. [Daredevil (1st series) #260]
Mary was wracked with guilt over Typhoid's actions, although she couldn't fully remember what they were consciously. Mary actually considered suicide, planning to kill herself in order to stop Typhoid's evil. She subconsciously chose the same bridge Daredevil was thrown over, however, and found him lying in the grass. The weakened Murdock was being attacked by one of the demons of Limbo due to Inferno and Mary wanted to protect him. Only Typhoid had the power to do so, though, and so Mary inadvertently let her other side out. Typhoid quickly returned to the original plan and revealed to the injured Daredevil that Typhoid Mary was both women in one 
body. When Daredevil was brought into the hospital, Mary was there as Karen Page came to see him. She confessed to Karen that she knew Matt's real identity and they were in love. Although Karen didn't believe it initially, Matt called out Mary's name in his sleep, destroying his relationship with Page just as Kingpin and Typhoid has intended. [Daredevil (1st series) #260-263]