Formal:
* Disease: For serious, chronic, or infectious conditions.
* Illness: A general term for a state of being unwell.
* Complaint: A specific symptom or problem a person experiences.
* Affliction: A serious or chronic condition that causes suffering.
* Disorder: A medical condition characterized by abnormal functioning.
Informal:
* Bug: A minor illness.
* Sick: A general term for feeling unwell.
* Complaint: A specific symptom or problem a person experiences.
* Trouble: A general term for something that causes difficulty or discomfort.
* Problem: A specific issue or concern related to health.
Other:
* Malady: A disease or illness, often used in a literary context.
* Condition: A general term for a state of health or illness.
* Trouble: A general term for something that causes difficulty or discomfort.
The best word to use will depend on the specific context. For example, if you are talking about a minor illness, you might use the word "bug." If you are talking about a serious condition, you might use the word "disease."