If you look at a periodic table, you will see that most metals are located near each other. This means that they behave similarly and one can easily replace one another in a protein or molecule. This may result in health problems. For example, lead, a toxic metal, tends to get stored in the bones because it mimics calcium, a metal that is a major component of healthy, normal bones.
Another example of mimicry occurs in the "engine" of the cells. A molecule called "ATP" is one of the major energy sources for all of our cells. It has three phosphate (PO4) molecules in it. Phosphate contains the metal phosphorus (P). When it is metabolized it provides energy and phosphate (PO4) which is used all over the body.
Another metal, arsenic (As), can mimic phosphorus in ATP. When this happens, the product of the reaction is ADP + AsO4. The AsO4 is not useful, compared to PO4, because it cannot be used to produce energy.