I'm not certain how I would handle the situation with your manager; however, I would point out the importance of NOT working when you're unwell. It really is a serious patient safety issue.
Last year, I witnessed a nurse who was still recovering from Influenza A (yet expected to work) sneeze in a neutropenic patient's room. Two days later, the PT was febrile and died 1 week later from Influenza A. The PT had previously survived 5 cancer surgeries and considering her situation, was doing remarkably well - only to be brought down by something that could have easily been prevented. Of course, none of us can know for certain it was the nurse who passed the virus on - but the correlation is alarming, since the PT was isolated. In short - if you're sick - don't go to work. Even if you aren't contagious - you still have a duty of care to ensure you're thinking clearly.
All I can think of is: What if a patient died because I made a mistake due to being unfit for work? Could I live with myself?