Low immunity can occur at any time. Do not blame cold and flu alone. You could blame it on the whether alone and it's always in the air. There are certain signs and symptoms that leads to low immunity levels:

You are always falling sick
This could be the most noticeable sign ever. If you find something viral doing the rounds, you would be the first person to know. You get more ailments than that of people around you. According to certain research methodology, an average adult around 2 to 4 colds in an year. If you lack that, your immunity definitely would require a boost.

You get very easily stressed
Stress does not come always due to emotional factors. If you have low immunity levels, it might also hamper your mental health too.

You have very little stamina
The day might have just began, but do you often find yourself feeling low on energy, even if you haven't done much physical activity? Feeling listless, lethargic and wanting to sleep most of the time isn't good and is a sign of low immunity.

You get major sugar cravings
Do not think that sugar if you have got cravings for sugar you do have a sweet tooth or something. Sugar does affect your immune system and also leads some effect on the germ killing response of the white blood cells.