Dua is from the most beneficial types of healing, it is the enemy of illness and affliction - repressing it and treating it, preventing its occurrence, removing it, or at least alleviating it. It is the weapon of the believer as is reported by al-Haakim in his 'Mustadrak' from the hadeeth of Alee bin Abee Taalib (RA) from Rasulullah (Sallallahu alaihi wasallam) that he said, "Dua is the weapon of the believer, the pillar of the religion, and the light of the heaven and earth." - Abu Yala
Dua takes one of three positions against illness:
It is stronger than the illness and therefore represses it.
It is weaker than the illness and therefore the illness overcomes it and afflicts the servant, however, it is still possible to alleviate the disease.
They are of equal strength and prevent each other from happening in the servant.
Al-Hakim reports from the hadith of Aishah (radiAllahu anha) that Rasulullah (Sallallahu alaihi wasallam) said, "Caution is of no avail against the decree but performing dua benefits those things that have occurred and have not yet occurred. For indeed while the tribulation is descending the performing dua meets it and they remain struggling with one another until the Day of Judgement." - Hakim, at-Tabarani
He also reports the hadith of ibn Umar from Rasulullah (Sallallahu alaihi wasallam) he said, "Dua brings about benefits to those things that have occurred and those things that have not yet occurred. So devote yourselves to performing dua O servants of Allah!" - Mishkat al-Misbah
He also reports the hadith of Thawban from Rasulullah (Sallallahu alaihi wasallam) that he said, "Nothing but performing dua averts the decree and nothing but righteousness increases the life span. Indeed a person is deprived of provision due to performing sins." - Mishkat al-Misbah
Allah knows the best