Difference Between Acute and Chronic Illnesses

When engaging in healthcare for yourself or a loved one, you will often hear a condition being referred to as "chronic" or "acute." Depending on who you speak to, the definition for these terms can differ, sometimes significantly.

Many medical dictionaries will define "chronic" as an illness that persists for six months or longer and "acute" as one that persists for less than six months. Although these definitions may apply in specific circumstances, they don't always apply and often fall short in describing what you may be faced with if given a chronic or acute diagnosis.

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. 

The general definition of an acute illness is one that is expected to be brief. Typically, an acute illness is one in which symptoms develop quickly and resolve in less than six months. By comparison, a chronic illness will persist beyond this timeframe and tends to worsen over time.

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.) Others extend the definition to include developmental, functional, or visual disabilities that require ongoing care or management.

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 infection has cleared. 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) that are often detected early and treated before any symptoms emerge.

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

After all, an acute bout of the flu in no way compares 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. Doing so, however, only adds to the confusion as the thresholds are frequently changed from six months to three months or extended to a year or more.

Even pubic health authorities are not immune to these concerns. The U.S. Department of Health and Human Services (DHHS), 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 DHHS, "chronic" is used to describe a public health concern for surveillance purposes. With the CMMS, "chronic" broadly describes a disease of healthcare utilization purposes.

There is currently not 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.

The seemingly random ways in which "acute" and "chronic" are applied can often create confusion in a patient's expectations of a disease.

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

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