You will only be able to process special category data if:-
your employee gives their explicit consent;
needed to carry out rights and obligations under employment law, e.g. to ensure the health and safety of workers or for complying with discrimination laws;
the employee has already made the data public, e.g. on social media
to establish, exercise or defend a legal claim;
to protect the vital interests of the employee or someone else, where the employee is incapable of giving consent. This lawful basis is very limited in its scope, and generally only applies to matters of life and death;
for the assessment of a person's working capacity.