Intelligence varied by country due to disease?

I briefed through an article in The Economist that reviewed a study that suggests a correlation between IQ scores and the burden of (exposure to) disease.  However, I am not entirely sure this link exists, as it may simply be replicating a bias against taught intelligence, or disparities of testing as it relates to accessibility of testing and quality of translation of test materials.  For instance, nearly every African nation has an average IQ score in the mid 60 to upper 70 range, and then Sierra Leone's average IQ score is 91.  Hard to believe that a nation that is at war with itself funded by blood diamonds, would score an average IQ of 91, don't you think?

Nonetheless, I did notice another interesting thing.

When you rank nations with the highest average IQs by their scores, the type of health care appears to matter:

• Singapore - 108
  
• South Korea* - 106
  
• Japan* - 105
  
• China - 105
  
• Italy - 102
  
• Mongolia - 101
  
• Iceland - 101
  
• Switzerland* - 101
  
• Austria* - 101
  
• Britain - 100
  
• Netherlands - 100
  
• Luxembourg* - 100
  
• Norway - 100
  
  
• US** - 98
  

* - Compulsory health care coverage required with a mix of private/public plans available.
** - Compulsory health care coverage not yet required.