The experience of having bloodshot or red eyes often accompanies general sicknesses like the common cold or the flu. This visible sign indicates the body is actively working to combat an internal challenge, even if the primary site of infection is not the eye itself. Red eyes are a common symptom of a systemic illness, manifesting complex biological processes. Understanding the underlying physical and immune responses clarifies why this symptom occurs when you feel unwell.
How the Body’s Immune Response Causes Redness
When a virus or bacteria invades the body, the immune system mounts a widespread defense known as systemic inflammation. This protective reaction eliminates foreign invaders and repairs damage. This systemic response triggers the release of chemical messengers, including inflammatory mediators like histamine, throughout the bloodstream.
These mediators act on blood vessels, causing them to relax and widen, a process called vasodilation. In the eye, this action affects the tiny capillaries that lie just beneath the clear membrane covering the white of the eye, known as the conjunctiva. The dilation and swelling of these capillaries results in conjunctival injection, which is the visible redness.
The goal of increased blood flow is to rapidly deliver immune cells, such as white blood cells, to the areas where they are most needed. Although the main infection might be in the throat or lungs, the eye’s rich blood supply is affected by circulating inflammatory signals. The increased volume of blood flowing through the widened vessels makes them more prominent against the white sclera, creating the bloodshot appearance.
The redness is a side effect of the body efficiently mobilizing its defenses to fight off the illness. This mechanism of increased blood flow and vasodilation is a general principle of the immune response, explaining why red eyes occur even when primary symptoms are respiratory or gastrointestinal.
Specific Infections Linked to Eye Redness
In many cases, the illness directly targets the eye’s surface, leading to a localized infection that causes redness. The most common condition is conjunctivitis, often called pink eye, which is the inflammation of the conjunctiva. This can be caused by viruses responsible for colds and upper respiratory infections, such as Adenovirus.
Viral conjunctivitis is highly contagious and typically produces a watery, clear discharge along with redness, often affecting one eye before spreading. Systemic illnesses like Measles also frequently involve the eyes, with conjunctivitis being an early sign appearing alongside fever and a runny nose. The inflammation in these cases results directly from the virus replicating on the ocular surface.
Bacterial conjunctivitis, while less common than the viral form, is another direct infection that causes redness. This type is characterized by a thicker, colored discharge, which may be yellow or green, and can cause the eyelids to stick together after sleeping. Differentiating the type of discharge helps medical professionals determine the underlying cause and appropriate treatment.
Other Contributing Factors and Warning Signs
Beyond direct immune and infectious causes, the physical strain of being sick can contribute to eye redness. Intense coughing, forceful sneezing, or vomiting can momentarily increase blood pressure in the head and chest. This rapid pressure spike can cause a tiny capillary on the eye’s surface to rupture, resulting in a subconjunctival hemorrhage.
A subconjunctival hemorrhage appears as a single, bright red patch on the white of the eye and is usually painless and harmless. Fever and general illness often lead to mild dehydration, which also affects the eyes. When the body conserves water, tear production can decrease, causing the tear film to become unstable.
Reduced tear quality and quantity can lead to eye irritation and dryness, manifesting as redness. The discomfort and fatigue that accompany illness may also cause a person to rub their eyes more frequently, physically irritating the delicate conjunctiva and exacerbating the redness.
While most cases of eye redness during sickness are temporary and resolve on their own, certain symptoms require immediate medical attention. Warning signs include severe pain, a sudden change in vision, or extreme sensitivity to light (photophobia). Redness localized to the iris, or the presence of a thick, persistent yellow or green discharge, should prompt a visit to a healthcare provider.