Diseases that can be fatal tend to frighten people.  Consider the ridiculous amount of news coverage that bird flu got.  Or SARS.  Diseases with a really short gestation period aren't a big problem^* because they don't spread very far.  The window of opportunity for getting the disease from someone who does not appear sick is small.  Diseases with a longer gestation period (like HIV) spread further, because the duration that the individual doesn't show any symptoms is longer.  But at some point, there's got to be a gestation period where something is no longer considered to be a disease.  If a hypothetical disease didn't show any symptoms for 100 years, people wouldn't worry about it.  Maybe there are diseases like this which have never been discovered, because there's no need.

GMO food won't be a problem once we know that particular strains don't give us any diseases that gestate shorter than some length (say 50 years).  That and those particular crops don't have other peculiar undesirable features like running around and killing all humans.

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^*Except for the unlucky individuals who catch them