Strong immune system does not mean lack of sickness but lack of prolonged symptoms of sickness and quick restoration.
Quick restoration is needed because one can not perform worldly duties under pain.
Only way to avoid sickness is to avoid planting seeds of sickness by performing actions/thoughts against your innate nature.
Entire world is now running after immune system. Eat this and eat that. Why? We are taught that the immune system is a system of biological structures and processes within an organism that protects against disease.
Can immune system really protect us?
No. They are actually cleaning staff in the body that are acting against imbalanced state and purging outcomes of diseases. Sickness was present since long, sprouted out of actions and thoughts against the nature of the body.
Immune system is working to reduce your pain/दुःख and attain सुख again so that you can perform duties. Because everyone’s righteous contribution is important for maintaining world order.
So who actually can protect us?
No one but self.
धर्मार्थकाममोक्षाणां शरीरं साधनं यतः।
Perform your duties and earn dharma, artha, kaam, moksha as per nature and constitution of your existence. Righteous actions will make sure that your existence is protected. Unless there is bad karma effect stored from previous birth.
Sickness takes birth from inappropriate actions and thoughts. Avoid them and follow your duties towards the world.