The central ideas of a text are those that are most important. The rest of the text is built around these main ideas. Main ideas also tend to be supported by details which expand on their information and claims. Finally, main ideas can be either explicitly stated or implied, without altering their importance and relevance to the text.
in this text illness means seasickness as mentioned but many people have this . even there is an illness in which people feel like falling down from high story which is dangerous which may cause death.