疾病 is a formal, written-term noun meaning 'disease' or 'illness' — referring to any condition that impairs normal bodily function, whether acute or chronic, infectious or non-communicable. The character 疾 originally meant 'sickness' or 'rapid onset illness', while 病 means 'illness' or 'ailment'; together, they form a compound that emphasizes medical seriousness and is commonly used in clinical, public health, and academic contexts.

Unlike more colloquial terms like 病 (bìng) or 感冒 (gǎnmào), 疾病 carries neutral-to-formal register and appears frequently in news reports, medical documents, and official announcements. It’s rarely used in casual speech — you wouldn’t say '我有疾病' to mean 'I’m sick'; instead, you’d say '我生病了'. However, it’s essential for reading health-related texts and understanding discussions about epidemiology, prevention, and treatment.