Nobody knows the exact illness Helen Keller had. It could have been meningitis, which does cause hearing and vision problems in many people, without affecting brain function.

She had a teacher that was dedicated to teaching her and only her, all day long. Many people do not have that kind of attention on them throughout their education.

She may also have just been extremely intelligent, these people are somehow able to thrive even with so many setbacks. Imagine how much more she could have done if she never became deaf or blind.