風邪は万病のもと
(Kaze wa manbyou no moto; “A fever is the root of ten thousand diseases”)

Definition:

Even if a disease seems to be mild (“just a cold”), don’t just brush it off. Things can get bad if complications arise, so be on your guard and protect your health.

Breakdown:

We begin with the noun 風邪 (kaze), literally “wind-evil.” Here I’m translating it as “fever.” (See below for a discussion of why.) This noun is marked as the topic of discussion by particle は (wa), and the comment on this topic centers on the noun もと (moto), in this case “origin,” “cause,” “foundation.” The associative particle の (no) specifies it as the cause of number-noun 万病 (manbyou), “ten thousand diseases.”

Notes:

While 風邪 is commonly translated as “cold” i.e. “the common cold,” it has historically referred to a broad swath of illnesses that involve elevated temperature – including the mild fever that comes with a cold, but also influenza and even the fever brought on by malaria. Consumers of Japanese culture may have at times been puzzled by how a character can be rendered bedridden, or pass out in the middle of a conversation, from “a cold”; this would be why.

However, I’m assured that contemporary usage has developed a distinction, so go ahead and use 風邪 for a cold, インフル（エンザ） for the flu, and マラリア for malaria.

Moto may also be written in kanji as 本, 元, or 因 without any change in meaning or pronunciation. And as is often the case, the meaning of 万 is less its literal rendition than simply “a large number,” so some versions of the saying scale it back to 百 (hyaku), “one hundred,” without any significant change in meaning. Some versions also replace もと with 始まり (hajimari), “beginning.”

This phrase supposedly comes to us from the “Basic questions” section of an ancient Chinese text known as the Huangdi Neijing (Japanese 『黄帝内経素問』, Koutei naikyou somon)

Example sentence:

「ただの風邪かもしれないけど、念のため、有給を一日とらせてもらった。風邪は万病のもとだからね」

(“Tada no kaze kamoshirenai kedo, nen no tame, yuukyuu wo ichinichi torasete moratta. Kaze wa manbyou no moto dakara ne.”)

[“It’s probably just a cold, but just in case, I took a day of paid leave. A cold is the root of ten thousand diseases, after all.”]