malvarmumi

The -um here is doing the thing the -um article warns you about: it is not derivable. Malvarmumi does not mean to be cold, or to make something cold - it means to come down with a cold, the illness. Hold it apart from its neighbours: malvarmigi is to cool something down, malvarmiĝi is to grow cold, esti malvarma is simply to be cold to the touch. The noun is malvarmumo, a cold, and malvarmuma describes the sniffly state. English hides the same ambiguity behind one word, so the Esperanto set is arguably clearer once you have learned it.