患者

Illness, not the person.

Sick person (more general/less formal than 患者 in a medical context).

Customer/guest, not a medical patient.

Often confused with other words that mean 'person' or 'customer' in English.

Specifically refers to a person receiving medical treatment. It doesn't apply to customers in a store or people in general.

医者は患者を診察する。(Isha wa kanja o shinsatsu suru.) - The doctor examines the patient.

While related, 病気 means 'illness' or 'sickness', not the person who has the illness.

病気 is the condition, 患者 is the person experiencing the condition.

彼は重い病気にかかっている。(Kare wa omoi byouki ni kakatte iru.) - He has a serious illness.

This also means 'sick person', but it's a more general term and can sound a bit dated or less formal in a medical context compared to 患者.

病人 is a general term for someone who is sick, while 患者 specifically denotes a patient under medical care.

病院にはたくさんの病人がいます。(Byouin ni wa takusan no byounin ga imasu.) - There are many sick people in the hospital.

This word means 'customer' or 'guest' and is sometimes incorrectly used for a patient, possibly due to the English word 'patient' sometimes referring to someone enduring something.

客 is for customers in businesses, not people receiving medical treatment.

その店には多くのお客さんが来る。(Sono mise ni wa ooku no okyaku-san ga kuru.) - Many customers come to that store.

The most general word for 'person'. While a patient is a person, using just '人' would not convey the medical context.

人 is simply 'person'. 患者 specifies a 'medical patient'.

彼は良い人です。(Kare wa yoi hito desu.) - He is a good person.