Natasha is, well, she is human mostly. If you look at it that way. She learns, she trains, she ages - slightly slower than most but she does age - she bleeds, she scars, she dies. But she comes back. The first time on a table in a cold hospital, eight years old, with a gasp and the first breath hurt her so much but she doesn't cry. She's not allowed to cry, it's a sign of weakness and the Red Room doesn't allow for weakness. The surgeons turn to the man watching and nod. It's the only time she sees him but she learnt his name, Dr. Randall Dowling later, in the files at SHIELD.
She tries not to die too often. The work he did; to her, to the people in Science City Zero, the unknown number of hospitals in Russia with the Red Room, is complex and even SHIELD is still trying to untangle the traces - the earlier versions, like the actress who loved inappropriate men, drank too much, talked too loose and paid for it, with a bullet and a radioactive half-life, are gone and she doesn't know if she has a finite number of deaths in her, if one day, she won't wake up, with the burn of life dulled from repetition in her chest.
But she does die some times; she's bled out more times than she cares to remember. Tortured twice, buried under the shattered concrete of a high rise building at least once. They shot her the first time, eight years old, back against the wall outside. SHIELD knew, or rather, suspected but Hawkeye didn't know when he made his call and didn't put an arrow through her throat. She bleeds out once on him later, in the basement of a mostly stable hotel somewhere in the Baltic states, his hand squeezing the makeshift dressing into the wound on her side. He called her Tasha, hissed through his teeth, kept telling her to keep looking at him, the evac would be here soon. She'd smiled at him, through bloody teeth and lips, said zhdat and let the darkness come.
She gasped awake in the back of the evac chopper with the sharp point of an arrow under her chin and 12 guns pointed at her. He was hurt that she didn't tell him, didn't trust him, but after the debriefing and the medical tests, she finds him on the roof and tell him that she does, tells him about being eight years old and how the bullet had burnt the life out of her and how life burnt when it came back in.
They've been partners now for a long time and if it comes to the end, if she doesn't wake up again, she'll be happy if the last time is in his arms.