Well.... susceptibility to flu/colds/infections might also signal a low Vit D status. Vit D has one function besides others of improving immunity.

Depending on your diet, whether you have Crohn's disease (which causes malabsorption) or Celiac/gluten intolerance (also causes malabsorption), the answer you seek my lie there.

B12 is the most delicate B vitamin however, and requires animal sourced food, adequate acid in the stomach, intrinsic factor from the stomach to latch onto the B12 from animal sources, and then trancobalamin in the intestine to carry it around. The other B vitamins are easier to fix and get from food.

B6 and folic acid have to be activated, for actions, and that may fail. Folic acid is methylated to methylfolate...and this like B12 may fail if you have an MTHFR mutation.

Some people get low in magnesium, and also in Omega-3 fatty acids. So if you don't eat flax or salmon (or other fatty fish) you can be low in Omega-3's. If you supplement magnesium don't get the OXIDE form as it is not absorbed in the bowel. Get a chelated form like, glycinate, malate, citrate, gluconate, etc.

So alot depends on lifestyle in addition to your genetic heritage.

Personality may drive food choices, poor sleep habits, and also affect immunity if it tends to be depressive. If you try too much body building and ignore diet, magnesium, B6, amino acids, omega-3's then you may deplete yourself while building muscle.
B12 is mostly for the nervous system, brain, and blood forming bone marrow. But it does work in all cells of the body to some extent. You cannot live without B12...but you can see from the RDA of 2-4 micrograms needed daily that it works in tiny amounts compared to other nutrients. And some is stored in the liver for lean times, up to 5 yrs depending on the person.