Mutants
Mutants are just humans who have evolved in a beneficial way, meaning it didn't harm them and the gene was allowed to stay with in the pool. They are almost the main breed of heroes and villains in Marvel. Mutants have been around since the start of time, being created by off world Celestials. Many Mutants have been discovered from times pre-dating the time period in which Egypt was at the height of its empire. Many mutants started to come into the picture shortly after WW2 because the nazi's had been breeding the X gene in them to create weapons, having no idea they would be destroyed and the prisoners were released into society to breed and pass on the modified chromosomes.
Many organizations have been created since WW2 but the most well known of them would be Charles Xaver's School for the Gifted and his team the X-men. The reason these havens needed to be created was due to the massive amount of hatred from the outside world toward the next step in human life.
Mutant Powers vary in a large way, the mutations having altered each person differently. Some mutants have the ability to teleport while others can control certain elements or change their anatomical structure, or have incredible healing abilities.
Being that their Powers vary it goes without saying that their weaknesses do as well.