Reply legal notice and attach your medicals. illness is a valid defence for not performing your part of contract.
The legal issue surrounding an employment bond depends heavily on the terms and their intent. The generally accepted position is that if the Company has incurred costs on training you, and the same is recoverable upon your leaving within a particular period, the Company may be entitled to pro rata reasonable costs. However, this is not set in stone and the position depends extensively on the facts. The most important document would be the employment agreement and the terms of the bond executed by you.
Hi. Per se the employment bond is legal in India. The requirement is that it should not be violative of Section 27 of Indian Contract Act. Further, the bond should be reasonable, clear and unambiguous. I would suggest that you do not send any reply to the legal notice as of now. In case the company proceeds to take any action, the same can be challenged in the court that the bond was unreasonable and contained unfair terms.