Red eyes, medically termed scleral injection, frequently occur when the body is fighting a systemic illness like a cold or the flu. This common symptom is a visible sign of the immune system’s widespread response as it attempts to eradicate a pathogen. While the redness can be alarming, it is typically a temporary side effect of the body’s internal battle against infection. Understanding how sickness impacts the eyes helps differentiate between a harmless temporary symptom and a complication requiring medical care.
The Core Mechanism: Inflammation and Vasodilation
The primary reason for red eyes during a systemic infection is the body’s inflammatory response. When a virus or bacteria is detected, the immune system releases chemical mediators, such as histamine, throughout the body. These potent compounds signal local changes, targeting the tiny blood vessels (capillaries) that run through the conjunctiva.
The chemical mediators cause these vessels to widen, a process known as vasodilation, which increases blood flow to the area. This increased blood flow delivers immune cells to the eye’s surface, making the normally inconspicuous capillaries larger and highly visible against the white sclera, resulting in the characteristic bloodshot appearance.
Physical Stressors That Worsen Redness
Beyond the systemic immune response, the physical symptoms of being sick can mechanically worsen ocular redness. Forceful actions like coughing, sneezing, or vomiting create a sudden and significant spike in pressure within the chest and head. This intense pressure surge momentarily stresses the delicate blood vessels on the eye’s surface.
In some cases, this abrupt pressure increase can cause a small capillary to rupture, leading to a subconjunctival hemorrhage. This bright red, localized patch of blood on the white of the eye is usually harmless, similar to a bruise. Excessive rubbing of the eyes, which often occurs due to irritation or fatigue during illness, also physically irritates the conjunctiva and contributes to generalized redness.
Localized Infections That Accompany Sickness
Red eyes can also signal a localized eye infection that develops alongside a systemic illness, most commonly conjunctivitis, or “pink eye.” Viral conjunctivitis is frequently caused by the same pathogens responsible for the common cold, meaning the eye infection can be a secondary manifestation of the main illness. This localized inflammation of the conjunctiva causes the small blood vessels to swell and become red, often affecting one or both eyes.
Distinguishing simple redness from a true infection involves looking for specific discharge. Viral conjunctivitis typically presents with a clear, watery discharge. A bacterial infection usually produces a thicker, sticky, yellow or greenish discharge that can cause the eyelids to crust together upon waking. The ease with which sick individuals touch their face and eyes contributes to the spread of the virus or bacteria from the respiratory tract to the eye’s surface.
Signs That Require Medical Attention
While many cases of red eyes during sickness resolve on their own, certain signs indicate a more serious complication requiring professional medical assessment. Any sudden change in vision, such as blurriness or complete loss of sight, warrants immediate attention. Severe, throbbing eye pain or extreme sensitivity to light (photophobia) should also be evaluated promptly. Other urgent symptoms include seeing halos around lights, or a pattern of redness focused in a distinct ring around the iris, which can signal deeper inflammation within the eye.