Disease vs. Illness: What’s the Difference?

When people talk about their health, they often use the words “illness” and “disease” interchangeably. Maybe you’ve “got an illness” or your provider diagnosed you “with a disease.” But do these words really mean the same thing — and why does it matter?

Turns out, these terms have different meanings. And knowing the difference can help you better understand and advocate for your health. Internal medicine physician Manjaree Daw, MD, explains.

What’s the definition of a disease?

A disease is a disorder that interferes with your body’s normal functions.

“Typically, a disease is something that your physician diagnoses, using a combination of your medical history, physical exam and test results,” explains Dr. Daw.

Although the word “disease” sounds serious, it’s not necessarily a life-threatening situation.

“Diseases can be minor, temporary issues or serious, long-term conditions,” she adds. “Sometimes, a disease can’t be avoided, but other times, there are things you can do to prevent disease.”

Examples of diseases

There are thousands of known human diseases, and they’re often categorized based on their causes. Examples include:

• Autoimmune diseases. Your immune system is designed to fight off invaders like germs. But if you have an autoimmune disease, your defense system mistakenly attacks part of your body. “Autoimmune conditions can harm healthy cells, organs and tissue,” says Dr. Daw. “Examples include Crohn’s disease, lupus and multiple sclerosis.”
• Deficiency diseases. A deficiency disease can occur when your body doesn’t have the nutrients it needs to function. “Iron-deficiency anemia is a common deficiency disease,” she shares. Other examples include rickets (vitamin D deficiency) and scurvy (vitamin C deficiency).
• Hereditary diseases. These diseases are inherited, meaning they run in your family, and may be caused by genetics. “Gene mutations often cause hereditary diseases, such as cystic fibrosis,” explains Dr. Daw. “Diseases like heart disease and cancer can be hereditary.”
• Hormonal diseases. Your hormones are chemical messengers that control functions in your body. They require a delicate balance to do their job correctly. “If you have too much or too little of certain hormones, you may develop a hormonal disease,” she says. Examples include adrenal disease, female infertility, male infertility and thyroid disease.
• Infectious diseases. An infectious disease is caused by a microorganism that gets into your body. “Infectious diseases are caused by viruses, bacteria, fungi and parasites,” Dr. Daw says. “And you can catch them from other people or contaminated food, soil, water or animals. Examples include colds and flu, Lyme disease and salmonella.”
• Idiopathic diseases. If a disease is idiopathic, it doesn’t have a known cause. “Many diseases occur without a link to genetics, infection or other known factors,” she notes. “Idiopathic thrombocytopenic purpura and idiopathic pulmonary fibrosis are some examples of idiopathic diseases.”

What’s the definition of an illness?

An illness is an imbalance in your physical, mental or psychosocial sense of well-being. It’s based on how you feel and/or what you perceive.

“If you have an illness, something feels off or you feel unwell,” clarifies Dr. Daw. “It’s what makes you go to your doctor and they work with you to determine what’s causing it.”

Examples of illnesses

A headache is a common example of an illness.

“You might get a headache from stress or staring at a computer too long,” illustrates Dr. Daw. “You don’t feel well, and those feelings are real. But a headache alone isn’t a disease, and you can’t see it on an imaging test.”

That doesn’t mean an illness is unimportant.

“If you feel ill, your doctor is here to help,” she continues. “Maybe you don’t need treatment in the traditional sense, like surgery or medication. But you and your healthcare practitioner can work together to address it so you can feel better. And sometimes, an illness can be a symptom of a serious disease, so we don’t want to ignore it.”

Another example of an illness is fatigue, or extreme tiredness.

“Fatigue can be a symptom of a disease, like depression or anemia,” says Dr. Daw. “But you might also have fatigue because you’re working long hours and aren’t sleeping enough. Or maybe you’re caring for an aging parent and a young child and feel very stressed and overwhelmed. An illness like fatigue is something we need to investigate so we can try to figure out what’s causing it.”

Other examples of illnesses include:

• Pain
• Skin rashes
• Tinnitus (sounds or ringing in your ears)
• Trouble sleeping
• Upset stomach

A partnership with your provider is key

Whether you call it a disease or an illness, if it’s interfering with your life, it matters. And that’s why everyone needs a trusted medical provider in their corner.

“Your physician is your health advocate,” emphasizes Dr. Daw. “Whether you have great health and want to continue to have great health or you need complex medical care, you deserve to have the best possible quality of life. And we want to help you achieve that.”