Zinc

Zinc is used in about 100 enzymes (reactive proteins) as a catalyst. It is used for immune strength, protein production, DNA production and cell division (in fact, no life form could divide it's cells if it didn't have zinc), and wound healing (which requires cell division). It is essential for growth in the young. It is used for the senses of taste and smell (olfactory function). Sexually active men need a good supply. Excellent for colds.
The body does not store it, so it needs to be replenished regularly. Men should have about 11mg, and women about 8mg daily.
A deficiency leads to symptoms like growth retardation, loss of appetite, lowered immune function and in more extreme cases, hair loss, diarrhea, slower sexual maturity, impotence, hypogonadism in boys (small balls), sores on eyes and skin, weight loss, wound healing prolonged, taste (flavour!) becoming strange, and mental slowness.
An excess leads to nausea, vomiting, loss of appetite, stomach cramps, diarrhea, lowered trace copper, impaired iron absorption, immune weakened, and possibly genitourinary problems.
People with digestive problems, pregnant/breastfeeding people, older breastfed only infants, vegetarians, people with sickle cell anemia and alcoholics risk lowered zinc. High levels of iron or copper reduce zinc and visa versa. Zinc reduces the effect of antibiotics, and penicillamine (for arthritis). Some diuretics reduce zinc.

In food zinc is most famously found in oysters. And rightly so. Seafood is a great source generally. Crab, fish, lobster, beef, pork, chicken, cereal, whole grain, bran, cheese, yogurt, baked beans, cashews, almonds, chickpeas, kidney beans, and peas are good sources.