Diplomatic illness is when a person who works for the government, usually a diplomat or someone who represents their country, claims to be sick in order to avoid doing something that they don't want to do. Sometimes they do this in order to not attend a meeting or to avoid a difficult conversation or negotiation. It is not the same thing as a regular illness like a cold or the flu. Imagine if you really didn't want to go to school, so you pretended to have a stomach ache to stay at home. That's what diplomatic illness is like, but for adults who work for the government.