Subdomain and Category Question

Hi, just want to know something about subdomain and category.

I installed my wordpress in http://www.domain.com and I know that when i put a post in one category says “health” the post will be http://www.domain.com/health/page.html .

What if I want it to be health.domain.com/page.html ?
How to solve this?

Is this make a difference using a subdomain and using the wordpress category in term of SEO?

Thanks.