Why do some people experience symptoms for months after an infection?
Post-infectious symptoms are likely caused by an interplay of energy deficiency in cells, damage to the nervous system and a dysregulated immune system, but the precise cause varies from person to person and has not yet been fully clarified. If you continue to experience symptoms months after an infection, it is worthwhile to take them seriously and discuss them with your doctor.
Persistent symptoms after an infection are not exceptional. About 10% of people who go through COVID-19 still suffer from fatigue, shortness of breath, brain fog or sleep problems months later. And that pattern is not unique to coronavirus: after dengue fever, the intestinal parasite Giardia and other infections, researchers see the same phenomenon.
One of the most commonly found abnormalities is that the small energy factories in cells, the mitochondria, function less well after an infection. As a result, energy production falls short, the immune system and blood vessels become dysregulated, and oxidative stress accumulates. This explains why you can feel exhausted after the slightest exertion. Whether this is the direct cause or rather an accompanying consequence has not yet been definitively established.
The autonomic nervous system, which regulates all kinds of unconscious bodily functions such as heart rate and blood pressure, is damaged in some patients. In a large group of long COVID and ME/CFS patients, blood flow to the brain was clearly reduced upon standing, and more than half had damage to small nerve fibres. This produces symptoms such as dizziness, heart palpitations and the notorious 'brain fog'.
At the same time, an infection can reprogram the immune system for a prolonged period. Immune cells subsequently overreact to new stimuli, which sustains chronic inflammation. The gut microbiome may also play a role: disruptions in the composition of gut bacteria can make the gut wall permeable, allow bacterial particles to leak into the blood and thereby continuously activate the immune system. Through the connection between the gut and the brain, this can also worsen cognitive symptoms. This mechanism is biologically plausible but has not yet been definitively proven.
In the case of symptoms following a bacterial infection, such as post-Lyme syndrome, there is no evidence that dormant bacteria are to blame. Additional courses of antibiotics do not help in such cases. The true cause remains unclear there. Women appear to face a higher risk of prolonged symptoms than men across several infectious diseases.
The claims are based on observational studies, mechanistic hypotheses and several larger patient cohorts (including 143 long COVID and 170 ME/CFS patients, and 1470 dengue patients). Direct causality has not yet been proven for most mechanisms. No large randomised trials are available on the causes of post-infectious syndromes.