In the US employment is "at will" which means you work at the whim of your employer (absent a union or specially negotiated executive contract). That means your employer can fire you for no reason, any reason or a bad reason as long as he does not fire you for an illegal reason. I know of no provision of law that an employer is obligated to keep on a non-functioning employee.
It sounds like you likely have a "serious health condition" for which the law may allow protected leave. Depending on the number of employees employed by your employer and how long you have worked for that employer, you may have rights--including the rignt not to be penalized for taking covered leave--under the Oregon Famly Leave Act ("OFLA"), which applies to employers with 25 or more employees and/or the federal Family and Medical Leave Act ("FMLA"), which applies to employers with 50 or more employees within 75 miles of the worksite. You must have worked for the employer for at least 180 days to qualify for leave under OFLA and 12 months to qualify for leave under FMLA. Leave under either is for no more than 12 weeks. Both employers and employees have certain duties under these leave statutes. I would suggest checking with the Oregon Bureau of Labor and Industries ("BOLI") www.boli.state.or.us for more information about your rights. If you qualify for leave, you should then request the appropriate paperwork from your employer to request covered leave.
If you have a right to leave and are fired or disciplined for requesting it or for utilizing it (including if your protected absences are held against you in performance reviews, etc.) you may have a statutory claim.
I hope this helps.