Whilst I'm no medic, I'm pretty sure you can attribute pychosomatic elements to a lot of illnesses. That is illnesses can be made worse by the mental state of the patient or even be caused by it. So, a cheerful doctor tells you you're going to recover, no problem. You trust the doctor (he's an expert) so you go away with a smile on your face with your prescription. You get the medicine / placebo. Whichever you get, you've now perked up after the doctor, so you soon start feeling better and recover.
2 examples:
I'm pretty sure a placebo would have zero effect if antibiotics were needed to fight disease. Your immune response is struggling to overcome a species of bacteria - that's where the wonders of antibiotics come in.
You're overweight, stressed, a heavy smoker and you go to the doctor with chest pains, knowing that your blood pressure will be too high (sphygmomanometers are not that expensive). He gives you blood pressure pills, so you walk away less stressed with less 'in need of' a cigarette (so you smoke a little less). He could have given you placebos here, quite easily. By reducing your stress and smoking, your blood pressure falls to safer levels.