An illness is a condition that makes your body or mind not work the way it normally does. When you have an illness, you might feel sick, weak, uncomfortable, or just not like yourself. A cold is an illness that makes you sneeze and cough. The flu is an illness that gives you a fever and makes your muscles ache. Diabetes is an illness that affects how your body processes sugar.

Some illnesses last only a few days, like a stomach bug that keeps you home from school. Others last much longer: if your grandmother has arthritis, that illness might affect her joints for years. Some illnesses are contagious, meaning they spread from person to person through germs. Others, like asthma or allergies, aren't contagious at all.

The word illness focuses on the experience of being sick, while disease is a medical term for what causes it. You might say “I have an illness” when describing how you feel, but a doctor might say “You have strep throat” when explaining what's wrong. People also use illness to describe mental health conditions that affect how someone thinks or feels. When someone has a serious or lengthy illness, we might describe them as ill rather than just “sick.”