ailment

An ailment is an illness or sickness that you have at any given time. You might say "I'm feeling unwell due to an unexpected ailment."

When we talk about someone's health, we often use words like "sickness" and "illness," but the word "ailment" can be a good choice too. If your friend has been experiencing stomach problems for weeks, it could be described as a long-term ailment that they're trying to overcome. The word is more general than "disease" or "condition," so you might use it if someone has an ongoing problem with their health, but also if something is temporary like a cold or flu.