There is a learning disability called dysgraphia, which can involve your ability to recognize numbers. Many other learning disabilities can also impact memory and thinking, and processing. Some diseases do affect mental function. Just because you are bad at math does not mean you have a disease.

It is estimated the between 3 and 6% of the population suffer from dyscalculia (http://en.wikipedia.org/wiki/Dyscalculia), a mental condition akin to dyslexia. However, I believe that for most people the trouble comes from (1) not taking the time to really master the basics (addition/multiplication of whole numbers,fractions, signed numbers as well as solving problems involving/using ratios and percentages) and (2) not taking the time/putting in the effort to really comprehend the concepts. You must be prepared to work hard on your own, and then to ask questions to complete your understanding, and then to practice so that the skills become automatic. Conceptual understanding without skills is useless, while skills without conceptual understanding is barren.

It is highly unlikely that problems with the subject math indicate a disease. There could be certain conditions which might inhibit or inhance a person's performance in math, but this would hardly be the only symptom. Many people are not great in mathematics and are completely healthy. A poor grade and/or understanding of math in and of itself is not an indication of disease.