There is more information about disease on Wikipedia
Disease is any abnormality in the body or mind of a creature that causes discomfort, distress or dysfunction.
In Space stage Edit
Diseases appear in the space stage. They spread through out the ecosystems and it is the player's job to destroy or remove the diseased individuals. If this is not done, the whole ecosystem can collapse. Individuals with diseases are marked by a glowing yellow trail, and others not affected by the disease will not. Sometimes diseases are called biodisasters.
Though you do not get diseases, if a herbivore eats meat or if a carnivore eats fruit your creature will vomit shortly after swallowing. No permanent or temporary side effects have been found, and it seems it has no effect on the player's creature. Also, if your creature's hunger meter gets too low you will start to lose health, as in starving.