Muzan was born in Japan's Heian era, to a constant struggle to survive on account of his frailty. His heart stopped multiple times in his mother's womb, and when he first entered the world, he was deemed stillborn, having no pulse and not breathing.
Muzan was diagnosed with a very rare unnamed illness and should have died at the age of 20. His doctor then traveled all around Japan and found the mystical Blue Spider Lily. Using it as part of his medicine seemingly cured Muzan, but the doctor's cure was incomplete and Muzan still suffered occasional relapses.
A millennium ago, during the Heian Era, Muzan was turned into a demon during an experimental treatment trying to cure his terminal illness, and his goal since then was to live without the fear of death and become truly eternal.
As stated by Kagaya, the disease currently ailing him is the curse inflicted on his family due to them birthing the "Original Demon".
2/10 His Body Is Weak To Certain Poisons
Poisons that did not exist before, like Tamayo's concoction of turning demons into humans, are the kind of poisons that Muzan's body isn't prepared for, as he's never been exposed to them before, and thus, his body has no idea as to how it should fight those poisons.
So the only reason Muzan is afraid of Tanjiro is because of the Hanafuda earrings he possesses, which acted as a reminder of his downfall. nxt. Hanafuda Earrings are a symbol of Sun Breathing users, traditionally passed down from generation to generation as a family heirloom.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank.
reason why Nezuko can't heal him because his illness is . not based on some demons blood demon. art. It's actually a curse the Ubuyashiki family is stricken with because Muzan.
Over a century ago, Doma held the position of Upper Rank Six ( 上 じょう 弦 げん の 陸 ろく , Jōgen no Roku?), and during that tenure, he turned Gyutaro and Daki into demons and introduced them into the Twelve Kizuki.
Curse of Kibutsuji
This curse besotted any who partake of Muzan's blood must be careful. The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo.
As a young man, Muzan was diagnosed with a disease that would kill him before he reached the age of 20. The doctor treating Muzan gave him a special prototype medicine, but it appeared to have failed. Enraged by his worsening condition, Muzan killed him.
However, she wasn't immune to the sunlight initially, as she got burned badly at first. So, in conclusion, her coming from a Sun Breathing user's lineage and not relying on human substance are perhaps the reasons for her immunity.
It is revealed that Tamayo's drug had an aging potion mixed in along with the Demon antidote. Due to this, Muzan has aged 9,000 years within the span of three hours.
While it is not explained in the anime why he did so, Muzan is shown to be living a normal life as a human. Not only that, he's married a woman and has a child with her.
Being a demon, Muzan is capable of living for many, many years. However, unlike other demons that have aged over hundreds of years. Muzan is well over 1000 years old. Despite achieving an age that no one else has, his appearance makes him look like an adult man in his early to mid-30s.
Advanced Demon Form: Nezuko has the ability to enter a berserk state where she becomes more demonic in appearance, transforming into an adult version of herself with vine-like tattoos that circle her body, a single horn protruding from her right forehead side.
Nezuko is at least stronger than Daki, meaning that if Muzan could bring her back under his control, she would certainly qualify for the most prestigious title a demon can hold: a member of the Twelve Kizuki. Thanks to Nezuko's remarkable strength, she would have to be an Upper-Moon.
It was then revealed that Muzan and Kagaya are from the same family which led to the Ubuyashiki family being cursed where every child born will be weak and die right away.
Yoriichi lived until around 85 years, so: 1432 + 85 = 1517, exactly 400 years prior to when Kokushibo remembered and died.
Technically, the elder of the two Sakuras` was actually Muzan`s child, by way of his mistress (Lady Jade Wong), who had one of her eggs inserted into Anne`s body.
The first demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with a disease which would kill him before he turned twenty.
Fun fact: The reason why Enmu was able to say Muzan's name out loud without dying is because they're married!
Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon. More specifically though, was the fact that Nezuko was of Kamado blood.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.