The main barrier which prevents the entry of disease-causing organisms into our body is

Which of the following are intimately related?

How has the discovery of antibiotics helped mankind in the field of medicine?

What is Louis Pasteur famous for?

What is a disease? What are the further classifications of disease?

$A$ - Acid in the stomach and saliva in the mouth are examples of physiological barriers.
$R$ - Tears from eyes are an example of a physical barrier.