The word "healthy" can have radically different meanings. An average person would probably take "healthy" to mean "not sick", while medical workers are more likely to consider someone healthy if they don’t have any medically-defined diseases or impairments.
The definition of "healthy" has evolved over time, and the fact that people usually evaluate their health subjectively makes the term "health" even harder to explain. Because of this, the World Health Organization has broadly defined health as "a state of complete physical, mental, and social well-being, and not merely the absence of disease or infirmity".
The trouble with terminology
We’ll start with the broadest, least specific term – medical condition. This umbrella term can encompass almost every possible aspect of a patient’s physical or mental health that requires some sort of medical care. With that in mind, the term "condition" may include any disease, illness, injury, genetic or congenital defect, pregnancy, childbirth, or any other biological or psychological state that lies outside a range that is considered normal and age-appropriate.
Moreover, “condition” is also used to indicate a non-critical state, as in "good condition".
But it gets even more complicated. The DSM (a psychiatric manual that defines all known and well-studied mental disorders) and some health insurance policies exclude mental disorders when regarding the term "condition". On the other hand, because of its neutrality, the term "condition” is sometimes used if stronger words, like "disease" or "disorder", might be less socially acceptable.
You go to a doctor with an illness, and return home with a disease
If a condition affects normal function, involves a structural change of a particular body part or organ system, or has specific characteristics (signs and symptoms) — if it can be diagnosed and isn’t caused by an immediate external injury — it is called a "disease".
Some people would say "illness" or "sickness" instead. Remember, however, that someone can be diagnosed with a disease without any symptoms (asymptomatic disease), and therefore doesn’t feel ill.
The term "sickness", on the other hand, adds a social element to this story. You are sick if you don’t feel healthy to a point where it affects your everyday abilities. You may not be able to go to work, and may even need someone to take care of you.
The cultural concept of sickness is worth mentioning, because it can lead to stigmatization, social rejection, and otherwise negatively impact a person’s life.
Long story short – diseases are cured, and illnesses are managed. However, it’s not that simple. Although a disease may cause illness, there are still diseases where the patient doesn’t experience any illness. You may also present with some sort of illness (such as chronic pain), where no structural damage can be diagnosed, and therefore it can’t be called a disease. A conundrum, we know!
"Symptoms are the body’s mother tongue; signs are in a foreign language"
After taking note of a patient’s reported symptoms, the doctor examines the patient to objectively detect medical signs of the patient’s condition. The doctor tries to evaluate the patient's state, while also looking for the origin of reported symptoms.
These two terms may overlap, and sometimes they can be treated as the same, but the general idea is that a medical condition manifests itself via subjective symptoms, which, when detected and objectively confirmed by a doctor, are referred to as signs.
Take manifestations of autism, for example. Although the more correct term for behaviors observed in this disorder would be "signs", because they are diagnosed by another person, the DSM classification instead uses the term "symptoms". This is most likely due to the fact that "signs" are objective by definition, and cannot be measured and quantified just like that in these patients (as in, you can objectively verify whether a bone is broken, but not how someone feels).
When discussing a mental disorder, we may define it as the coexistence of particular emotions and actions which form a distinguishing pattern. However, it is also known that different causes can manifest with the same signs or symptoms, while on the other hand, a set of symptoms may be named a syndrome, even if they aren’t a part of a disease, like PMS.
This may also depend on who you ask. Some will tell you that a "syndrome" can only be called that in the absence of a direct known cause of a patient's signs and symptoms. If the cause is identified, it ceases to be a syndrome, and can now be characterized as a disease, for example. Geneticists, on the other hand, use "syndrome" only if the cause is known, whereas they use "genetic association" if they only suspect an underlying cause.
Medical terms: Form over function, or function over form?
When a physical or mental condition only leads to a functional abnormality, it is called a "disorder". Although somewhat interchangeable with the word "disease", and especially "illness" when talking about mental health conditions, the lack of structural changes in a certain condition implies that "disorder" sounds less restrictive, and therefore, more desirable to use instead.
Besides the ones previously mentioned, other disorders are classified as physical, genetic, functional, or metabolic, usually stressing the notion that this type of condition isn’t caused by infectious organisms.
For example, dyslexia is, by definition, a disorder, although you wouldn’t make a huge mistake if you called it a condition. The same goes with autism. However, because autism can manifest with an extensive range of symptoms, skills, and severity, it is correctly referred to as an autism spectrum disorder.
This is why the more correct and precise term for sexually transmitted diseases (STDs) would be sexually transmitted infections (STIs).
It doesn’t mean the same to me!
We’ve seen that "healthy" is incredibly hard to define. The same may be said for "normal", by which you assume that something is standard, typical, or expected. Therefore, "abnormal" would mean the opposite. However, this term should only be used in a medical context, because describing a person as abnormal would imply that they are somehow deviant, dehumanizing and stigmatizing them in the process.
Abnormalities that happen to be present at birth are called "birth defects", where the word defect implies imperfection, or the absence of some structures. Other than this use, the term "defect" should not be used when referring to a patient.
Instead, we may discuss "disability", another umbrella term describing functional limitations that affect regular daily activities, thereby presenting a more complex issue than just a health problem. However, it is always a good idea to specify the exact type of disability in question. Similar goes for "deformity", which implies that a part of a body doesn’t have a typical shape, unrelated to the underlying cause.
Take HIV, for example: a person is infected with it, and because HIV weakens the immune system, it can eventually lead to an array of other conditions with their own signs and symptoms, which are altogether characteristic and specific for HIV. As of then, it’s no more called HIV infection, but AIDS – acquired immunodeficiency syndrome. And although it checks all of the boxes for it to be called a disease, to protect the patient from the burden of this word, usually the more neutral-valued term, such as condition, is used.
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