The Difference Between Acute and Chronic Illnesses

Broadly speaking, acute conditions are those that occur suddenly, have immediate or rapidly developing symptoms, and are limited in their duration (e.g., the flu). Chronic conditions, on the other hand, are described as those that are long-lasting and develop and potentially worsen over time (e.g., Crohn's disease).

But these descriptions can vary somewhat depending on who you speak to or what sources you reference. While the terms may apply in specific circumstances, they don't always, and they often fall short in describing what you may be faced with if given an acute or chronic diagnosis.

General Definitions

Most illnesses can be categorized as acute or chronic. These terms can suggest the types of treatment required, how long treatment can be expected to last, and if treatment is appropriate. 

Acute does not mean new, although many newly diagnosed diseases present with acute symptoms. Nor does it mean that symptoms are severe. It simply means that symptoms have developed quickly and that some sort of medical intervention is needed.

Similarly, chronic should not be construed to mean fatal or something that will inherently shorten your life. It simply indicates that the condition is not curable but can often be managed (like diabetes or high blood pressure).

A newly diagnosed illness can also be labeled chronic if there is no expectation of a cure; arthritis is one such example. Some extend the definition to include developmental, functional, or visual disabilities that require ongoing care or management.

Phases of Illness

An acute or chronic diagnosis is not necessarily fixed. An acute condition can sometimes become chronic, while a chronic condition may suddenly present with acute symptoms.

Certain infections, for example, will progress from an acute phase (in which symptoms appear and resolve after the initial exposure) to a chronic phase (in which the infection persists, but progresses less aggressively). The chronic infection may lie dormant for years in a latent state, only to manifest with new and typically severe acute complications.

Syphilis and hepatitis C are two such examples. Both will typically present with acute symptoms that spontaneously disappear, suggesting that the infections have cleared. However, if left untreated, the infections can progress silently and emerge years later with severe complications like tertiary syphilis or liver failure, respectively.

The same can occur with non-infectious disorders like rheumatoid arthritis or psoriasis. Both are considered chronic in that they cannot be cured but can be managed with proper care and treatment. Even so, the diseases can have episodic flares in which acute symptoms spontaneously develop and disappear.

However, with early diagnosis and treatment, some chronic disorders may remain subclinical (without readily observed symptoms) and never manifest acutely. These include infections like HIV or conditions like hypercholesterolemia (high cholesterol), which are often detected early and treated before any symptoms emerge.

Where the Definitions Fall Short

As tidy as the definitions may seem—six months or more for chronic versus less than six months for acute—these timeframes in no way suggest what you may be faced with if diagnosed with an acute or chronic illness.

After all, an acute bout of the flu does not compare to an acute hepatitis C infection. Neither does HIV (a chronic infection that can be controlled over a lifetime with antiretroviral drugs) compare to multiple sclerosis (a chronic illness that invariably progresses despite treatment).

This non-specificity of definitions not only affects doctors and patients but also researchers who look for concise ways to evaluate the course of a disease. Thresholds are frequently changed from six months to three months or extended to a year or more, which only adds to confusion.

Even pubic health authorities are not immune to these concerns. The U.S. Department of Health and Human Services (HHS), for instance, lists 20 diseases as chronic —including stroke, autism, and cancer—while the Centers for Medicare and Medicaid Services (CMMS) lists 19, many of which are different.

Within this context, the definition can often bend to fit the situation. With the HHS, chronic is used to describe a public health concern for surveillance purposes. With the CMMS, the term broadly describes a disease for healthcare utilization purposes.

There is currently no one consistent definition of either acute or chronic that fits all purposes. This doesn't mean, however, that the terms are unimportant in how they are used between a doctor and patient.

Clearing Up the Confusion

The seemingly random ways in which these terms are applied can often create confusion in a patient's expectations.

For example, can cancer truly be considered chronic when only a few types (such as multiple myeloma) are able to be managed chronically? Should a traumatic injury like a broken leg be considered acute even if it fits within the broader definition of the term?

In the end, defining an illness or injury as acute or chronic may not only not be necessary, but it may confuse more than enlighten.

Some health experts advocate for a simpler approach to help clear up confusion and inconsistencies. Rather than adhering to a specific timeframe or list of conditions, they endorse definitions that express the concepts behind the terms more generally.

Merriam-Webster's dictionary, for example, defines them as follows:

• Acute: "Having a sudden onset, sharp rise, and short course"
• Chronic: "Continuing or occurring again and again for a long time"

By understanding the concepts rather than the rules, you can have a better grasp of what your doctor is telling you when describing your health condition. But, of course, be sure to ask any questions you have to paint a clear picture of your condition and what may lie ahead.