A person performing `Umrah and making Tahallul then being attacked by a disease preventing him from performing Hajj rituals

Firstly, with regard to your Ihram for `Umrah as a Mutamatti` (pilgrim performing `Umrah during the months of Hajj, followed by Hajj in the same year with a break in between) while ending the `Umrah rituals and there was no Ihram for Hajj owing to your incapacity to enter into it, there is no sin on you for this. Secondly, if your illness lasts and you cannot perform Hajj yourself and there is no hope for you being cured from this disease, it is permissible for you to delegate another person to perform the obligatory Hajj on your behalf. May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and his Companions.