The mercury in fish when consumed in too high of levels mainly affects the humans nervous system and can be deadly if you do intake too much mercury from fish.
The mercury in fish also affects humans digestive and immune systems, as well as lungs, kidneys, skin and eyes.
The mercury contained in fish mostly affects young children and babies still in the womb but it can still affect older kids, teens and adults as well.
It's very hard to eat enough fish to get mercury poisoning but it's still best to avoid eating too much fish to avoid getting high levels of mercury into your body.
Eating fish in moderation is very healthy though and can promote brain health and may even prevent dementia.