At a guess, I would say that a country with high levels of diseases would be less developed purely because this means that the country would have to spend a lot of money on healthcare to cure/treat people. However I suppose it would make a difference what disease it was and how developed the country was when they got it. For instance, Britain has problems with heart disease, but this is now more likely to be caused because of development, as it has encouraged a high fat lifestyle etc, but somewhere like sub-saharan Africa has a problem with AIDS, but was not developed enough to deal with it, therefore preventing the country from developing at the rate it once was.