It does sound as if you need an assistant or your boss needs two. 
Your health is your number one priority. 
I told a boss once, I have priorities. 
1.  My Family 
2.  My health 
3.  My job responsibilities. 
The first two I can not replace.  However, I can always find another job.  It may not be the ideal job, and it may take time to find it, but I will always find another job.  He respected me for that and we ended up working well together for a few years.

You should not be asked to work while out sick.  I would propose getting additional resources within the department to meet the demands of the increased workload required to meet the goals and objectives of your boss and the organization.  Whether  it be temps to assist, or a permenant placement.  You should not work when you are sick and on leave.  Pneumonia is not caused by a person, but obviously your system was worn down which weakened your immune system.  I recommend you propose the additional resources.  If that request is not granted, then you are well within your right to interview within the company for other jobs.  Does your company have an internal job posting system?  Check it out to see what jobs are available. 

You may want to consult with HR if you feel you can not come to agreement with your boss.  I too work for a "work- aholic"  he admits it.  But he does not expect me to do the same.  Yes, I have to put in extra time and be flexible to keep up and stay 2 steps ahead....within reason.   

Keep us posted and let us all know how it works out.