When Allah U increases their disease, it is not injustice from Him. This is because, they are the ones who chose straying from the right path, lying, wrong-doing and infidelity. Allah U says:

﴾وَهَدَيْنَاهُ النَّجْدَيْنِ﴿

“And have shown him the two ways” [Surah Al-Balad: 10] and

﴾إِنَّا هَدَيْنَاهُ السَّبِيلَ إِمَّا شَاكِراً وَإِمَّا كَفُورا﴿

“Indeed, We guided him to the way, be he grateful or be he ungrateful.” [Surah Al-Insan: 3].

 He gave the human being a choice. It is either he chooses the choice of disease and Allah U will add on to his disease due to his own choice, or he chooses health and safety and Allah U will guide him to achieve them.

If a doctor advises a sick person not to eat sugary foods so that the amount of sugar in his blood will not increase, but he continues eating it. He is the one at fault if anything happens as he is the one who chose to do so without being forced by anyone. This is exactly the case for the person who chooses to have the disease, thus Allah U will increase it in him.

﴾وَلَهُمْ عَذَابٌ أَلِيمٌ بِمَا كَانُوا يَكْذِبُونَ﴿

“and for them is a painful punishment because they [habitually] used to lie.”.