Ultimately you don't have to be correctly diagnosed to become successfully cured. As long as the medicine is effective and the symptoms disappear, you won't notice the difference. The breakout of any illness signals that your body's defences have temporarily gone down and something has made it's way into your body creating a disruption. Medicine aids the body by helping to repair these defences after which the body is expected to take care of the rest.
Chances are that the initial diagnosis was not wrong but merely incomplete. More than one disorder may have invaded and/or more than one disease may have broken out. Such a condition is called comorbidity. It is a common enough condition, but has only started receiving individual attention over the past years. For instance, hypertension will pose a significant threat by itself, but will at times occur together with diabetes. This will not always merge with hypertension, but can equally occur with kidney or liver diseases.
For instance, Taylor reported in 1998 that the survival rate of larynx cancer patients depends on the stage of the sickness and the possible presence of comorbidity. In the first stage of cancer development, the absence of other diseases makes a difference in survival rate of 83% versus 17%. These values drop to 50% and 0% respectively in the fourth stage. Overall, the comorbid patient shows a 59% lower expectancy of being cured than someone who is comorbidity-free.
Comorbidity is not restricted to physical subjects. We can see similar states occurring in psychiatry, psychology & mental health. What the main difference can be said to be is that in medicine the concomitance of two or more diagnoses is not deemed acceptable. The goal is for this to be realised, demonstrated and accepted as soon as possible. Early medication can only be regarded as acceptable where it helps to diagnose & cure.