When ill, many people notice their eyes appear red or “bloodshot.” This occurs because tiny blood vessels on the conjunctiva, the clear membrane covering the eye’s white part, become enlarged. While concerning, this is often a temporary and harmless response by the body to illness.
Inflammation and Eye Redness
The reason eyes turn red during sickness is the body’s inflammatory response. When an infection or illness occurs, the immune system activates protective mechanisms, including vasodilation. This process expands blood vessels to increase blood flow to the affected area. In the eye, vasodilation primarily affects the conjunctiva, a transparent membrane covering the white part of the eye and lining the inner eyelids. When its fine blood vessels become irritated or inflamed, they enlarge and become more prominent, causing the characteristic red appearance. This increased blood flow brings immune cells and other protective substances to combat the illness, a visible sign of the body’s defense.
Infections That Cause Red Eyes
Specific infections, such as viral and bacterial conjunctivitis (pink eye), frequently cause red eyes. These infections inflame the conjunctiva and can be part of a broader systemic illness.
Viral conjunctivitis is the most widespread form of pink eye and is highly contagious. It is caused by adenoviruses, the same viruses causing the common cold. Symptoms include a watery discharge, a burning or gritty sensation, and redness, often beginning in one eye and spreading to the other. This type of pink eye often accompanies cold symptoms like a sore throat or runny nose and resolves on its own within one to two weeks.
Bacterial conjunctivitis, less common than viral forms in adults, is caused by bacteria like Staphylococcus aureus or Streptococcus pneumoniae. This infection is characterized by a thick, pus-like discharge that can be yellow or green, causing eyelids to stick together, especially upon waking. Redness and swelling of the eyelids are common. Unlike viral conjunctivitis, which produces a watery discharge, the discharge in bacterial cases is thicker. Bacterial conjunctivitis is contagious and spreads through direct contact with eye secretions, sometimes requiring antibiotic eye drops for treatment.
Allergies and Other Contributing Factors
Beyond direct infections, other factors can cause eye redness during illness. Allergic conjunctivitis occurs when the immune system overreacts to airborne substances like pollen, dust mites, or pet dander. The body releases histamine, which causes blood vessels in the conjunctiva to swell, leading to red, itchy, and watery eyes and sometimes puffy eyelids. Unlike infectious conjunctivitis, this allergic reaction is not contagious.
Dehydration, a common side effect of illness due to fever or reduced fluid intake, can also contribute to eye redness. When the body lacks sufficient hydration, tear production can decrease, leading to dry eyes. This dryness causes irritation and a gritty sensation, resulting in redness. Tears lubricate the eye’s surface and wash away irritants.
Physical strain from symptoms like coughing, sneezing, or vomiting can temporarily increase pressure within the head and eyes. This pressure can cause tiny blood vessels on the eye’s surface to break, leading to a bright red spot known as a subconjunctival hemorrhage. Although alarming, this is a benign occurrence that resolves on its own without intervention.