I know it is against islamic law to put fake hair in. Now, what if you have a major illness and you want to look normal so others do not find out about this illness, is it still haram to wear fake hair?

All praise be to Allah and may His blessings and peace be on His last messenger, Muhammad,

The Messenger of Allah (blessings and peace be upon him) said,

” لَعَنَ اللَّهُ الْوَاصِلَة”

“May Allah curse the one who makes wasl (connecting [her hair with something else]).” (Al-Bukhari)

This applies by agreement of the four schools to the case of a non-married woman who lengthens her hair with human hair. This is particularly so if deception of a suitor is intended.

Wearing a wig for the sick (in front of women and mahram men) is permissible if the hair doesn’t belong to another human being (synthetic for example) and there is no deception involved, meaning she doesn’t wear it in front of a suitor to look different from her reality. This position will be in accordance with the hanafi position which allows non-human hair to be used, and the malikis, who allow the wig because they don’t consider it like the prohibited (wasl) which is lengthening one’s hair with fake hair by joining both, and the shafi’ees, who allow (wasl) with non-human hair for a married woman with the permission of her husband (if that is the case here), as well as the hanbalis who allow it with non-human hair for a need.

Allah knows best.