What Is a Subclinical Infection?

A subclinical infection is one that is present in a host who shows no outward signs of symptoms and cannot be diagnosed without testing for a specific infectious agent. Also termed "an inapparent infection," it can pose risks to the host and the surrounding population until it is resolved. People may carry infections for days, weeks, or even longer without being aware of it, unless they are tested when a medical provider suspects a problem, or if the test is part of a routine workup. Identifying and treating such infections is critical for personal and public health.

From the point of view of an infectious organism, there are a number of advantages to a subclinical infection. The organism can live inside the host, reproducing and getting stronger, and as the host moves around in society, the infection is passed on. Someone who carries an influenza virus without knowing it, for example, expels viruses with every breath or cough. People can pick up the infection, allowing it to travel out through the population.

For public health, of course, this is a significant problem. Infections can travel before practitioners are even aware of an outbreak. The presence of a subclinical infection can also make it hard to track the origins of an outbreak or epidemic. In vulnerable populations like patients with compromised immune systems, there are specific concerns about the risks of interacting with people who may carry infections without knowing it. Asymptomatic carriers can be anywhere, and may unwittingly pass on dangerous infections.

This can also cause a problem for the host. A subclinical infection may cause complications by weakening the immune system, making the host more susceptible to inflammation and infection by other organisms. Research on pregnancy complications, for example, shows that premature labor can sometimes be linked with a subclinical infection that wasn’t identified and treated. Consequently, pregnant women may undergo a thorough medical workup at the start of a pregnancy to identify specific concerns.

Some infections are notorious for their subclinical nature. People can carry herpes without being aware of it, for example. Routine physical examinations may provide an opportunity to collect samples so a medical provider can identify common infections in the patient. Infections can also be identified in a workup to determine why a patient is experiencing symptoms like inflammation and fatigue. When the infection is found in testing, it can be treated to eliminate or suppress it, depending on the organism involved.