Which word describes a condition that develops so gradually it is not immediately noticeable?

The term that best describes a condition that develops gradually and is not immediately noticeable is "insidious." This word is often used in medical contexts to refer to diseases or conditions that progress slowly and subtly, making their presence difficult to detect early on. For example, certain cancers or chronic illnesses can be insidious, quietly advancing without showing significant symptoms until they have reached a more advanced stage.

The other terms provided have distinct meanings that do not align with the concept of gradual onset. "Invasive" typically refers to something that tends to attack or invade, often used in the context of infections or procedures that penetrate bodily barriers. "Lethargic" describes a state of sluggishness or lack of energy, which does not pertain to the gradual development of a condition. "Intact" means something that is undamaged or whole, which does not describe changes over time. Therefore, "insidious" accurately captures the essence of a condition that is slow to become apparent.