When to get a doctors note if I might have to miss an exam? I've got a midterm tomorrow and have been increasingly ill for the past few days. I need to decide whether or not I'm going to write the midterm, and if I don't I'd need some sort of proof that I was in fact sick. My question is, how sick should one be to not write a midterm? It is possible I could push through it.

Any advice on when to get a medical note to be excused? The way I see it 99% of the time it is technically possible to at least be present for the exam; even if one has the flu or strep throat they can still write it (though poorly). The institution only asks for a medical certificate, but even unintentionally one could fake being sick for example if I go to the doctor and tell them I'm here to get a note, they would probably give me one even if I'm hardly ill.

When should someone make the call to stop doing the work and get the note? A couple of my recent assignments weren't done to as high a standard as I would've liked because the illness is making it hard to concentrate and work, but I thought I would get better in a few days. 

The other thing is I'm having trouble finding a doctor or nurse. I don't have a GP where I'm going to school and the campus medical is fully booked. I've phoned around to some drop in clinics and they have wait times of over an hour and charge at least $10 for a note.

UPDATE: I managed to get a note from a drop in clinic and scanned and e-mailed it to my professor. The course policy is that missed exams cannot be taken at a latter time and a mark for them will be calculated based on everything else in the class excluding the final exam.
Check your course syllabus.  Often, policies for exam makeups are documented there.  It also might say here whether you need a doctor's note or not.
Contact your professor right away (by email or phone) to inform them that you are ill and may miss the exam.  As a professor, the further in advance I know about situations like this, the easier it is for me to make arrangements.  You could also ask at this time about whether you need a doctor's note (or confirm what you read in the syllabus), and/or explain the difficulties you would face in obtaining one.  The professor may be willing to make an exception.
Obviously, go to the doctor if you are in need of treatment.  Also go if the professor insists on having a note; this is probably not ideal if you don't otherwise need to see a doctor, but you really don't have much choice.
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Only you can make the decision as to whether you are "too sick" to take the midterm.  My feeling is that any of the following would be sufficient reasons to stay home:

you think you would significantly risk your health by attending
you have a contagious illness
your illness might disturb other students (coughing, running to restroom, etc)
you think your illness would significantly reduce your ability to do well on the exam (e.g. you are in pain, excessively fatigued, have distracting symptoms, etc)