My colleague tells me that I usually get sick because I did not complete a year of breastfeeding from my mom. Is it true that breastfeeding affects the immune system forever?

It doesn't completely make a person invulnerable but to some extent, yes, it strengthens immune system even after weaning. This can be lifelong. Charge it to the bacteria that has been cultivated in one's gut which may live until the person dies. Of course, the healthier the gut, the stronger the immune system because gut wall is home to about 70 percent of our immunity cells.

It may be true that breastfeeding affects the health of a person for a lifetime but of course, it can be influenced by other factors. No matter how long a person has been breastfed, if his/her lifestyle is unhealthy, then I guess that can affect a person a lot.

I think it's somehow not true. My wife has been breastfeeding for 10 months straight and there were times that she got sick due to flu. Healthy lifestyle I think is still the factor to battle sickness whether breastfeeding or not.