As per the John Hopkins University, a big part of our immune system lies in our gut. Our gastrointestinal tract has good bacteria that release antibodies to fight infection. Thus, if there is an imbalance in the gut bacteria, you will become more prone to catching infections. It also makes one prone to autoimmune disease and various other inflammatory conditions.
Gut bacteria also plays a big role in digestion. So, if you get frequent stomach issues like diarrhoea or constipation, it can be a sign that you have a weak immune system.