An employee where I work is claiming I have caused him to suffer a medical condition (as yet unknown as even doctors don't know the cause yet), due to the stress I have put him under. I have not asked him to do anything that I have not asked every other person in his position to do, nor do I expect any more from him than others in the same position. He has also accused me of being racist and sexist with no basis and has physically intimidated me. His claims are causing me great distress and are starting to affect my own work. What can I do to stop him from spreading claims that his issues (overweight, unfit) are my fault without medical evidence to prove it, and if he continues to slander me by claiming that his current situation is my fault, can I sue him under Defamation Law? When it is found that I have nothing to do with his health issues, can I sue him then?