Symptoms of vitamin C deficiency: Slow healing of injuries, small mouth sores that don't heal, malaise, lethargy.
Symptoms of vitamin D deficiency: Fragile bones, muscle weakness or twitching.
Symptoms of iron deficiency: Anemia, weakness, sometimes pica (a desire to eat ice cubes, clay, or some other non-food item). They test for this when you donate blood, so you don't have to involve a doctor to find out if you're deficient in iron.
Other deficiencies are much less common (at least I don't know anyone who's had them).
Slight tangent, does "malaise" have a more specific meaning in this context? Google gives me "generalized feeling of discomfort, illness, or lack of well-being." which is unhelpful.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.