In addition, probiotics can help improve the function of your immune system and improve digestive function in general -- so I'm not surprised that you feel better after adding probiotics to your diet.

CNN: Can supplements block benefits of probiotics?

But their side-effects can upset the function of the body's immune system, which could be crucial in a severely injured patient, who needs to fight potential infection.

BBC: Heading for trouble

Isolation, loneliness and low social status can harm a person's subjective sense of well-being, as well as his or her intellectual achievement, immune function and health.

CNN: The importance of belonging

Beta carotene is a type of carotenoid, a plant pigment that is a provitamin for vitamin A. During its digestion and metabolism, a percentage is converted to vitamin A, a fat-soluble vitamin that is important for vision, growth and development, and healthy immune and reproductive function.

CNN: Should I take beta carotene as a supplement?