Firstly: 

There is a difference between one of the parties hearing from people that So and so has diabetes, which will have al ighter impact, and one of the parties telling the other about that directly, or the woman’s guardian telling the fiancé about that. This has a greater impact and is more off-putting, and may lead to an end of the marriage proposal.

 Secondly: 

The degree of severity of this disease may vary; if it is the regular type that many people suffer, there is no need to tell the fiancé(e) about it. But if it is chronic and serious, then the other party must be told about it. 

And Allaah knows best.