Hello,
Diseases like Diabetes, Asthma, etc are very common now-a-days.....like Nathrao mentioned, this doesn't qualify to avoid Notice period. And even if sometimes this needs hospitalization, the Company will only extend the NP by that duration.
Looks like you have some OTHER reason(s) to avoid the NP but are searching for something that you can get the Company to accept.
Also vis-a-vis your line "is it mandatory that i have to buy notice period?", pl note that you CAN'T take this for granted. Even if this option exists in the Rule Book, the Company [usually the Reporting Manager] will have the Final discretion.
When you join a job, it's NOT your prerogative to think or say "I dont want to serve notice"......you either ACCEPT the FULL deal or LET GO the FULL deal. If you want any changes in the terms, it needs to be done thru discussions.
What IF the Company says 'we promised you X salary, but want to give you X-Y salary from now'--would that be OK to you? Won't you quote Rules then?
Rgds,
TS