Medicine has begun only to scratch the surface of how infectious diseases can cause damage far beyond the initial symptoms. One well-established complication of a bacterial or viral infection is Guillain-Barré syndrome, an immune reaction against the body’s nervous system that can cause weakness and, in some cases, paralysis. Most people recover fully, but by some estimates it is fatal in around one in 20 cases.
One potential explanation for Guillain-Barré syndrome — which could apply to other complications — might have to do with a phenomenon called “molecular mimicry,” said Dr. Prathit Kulkarni, an infectious diseases specialist at the Baylor College of Medicine. If the pathogen’s proteins are similar in shape to those found in the human body, antibodies might accidentally react against a person’s own organs, Dr. Kulkarni explains.
In addition to turning the body’s immune system against itself, a microbe can cause direct and irreparable tissue injury or ignite damaging inflammation, which could account for later illness and persistent symptoms.
Parsing these connections is tricky, in no small part because a virus or bacterium may no longer be in the body even though its reverberations linger. “By the time you’re caring for the person, the pathogen is long gone, it’s just that it’s initiated this cascade of immune-mediated damage,” said Dr. David Fisman of the University of Toronto’s Dalla Lana School of Public Health.
In the case of Covid-19, researchers have many different theories about what might cause lingering illness. One idea is that the coronavirus itself directly damages tissues by killing cells, and affected organs never recover. Another idea is that the virus prompts inflammation that indirectly causes harm to organs. Some researchers suggest that long Covid sufferers might harbor a reservoir of the coronavirus. They have speculated that the Covid-19 vaccine might spur an immune response that eliminates these reservoirs, which might explain why some people with long Covid report feeling better after vaccination.
Addressing the gaps in understanding complications that arise from infectious diseases will take time and dedication. For starters, the medical community needs to establish a clearer consensus on what these complications are for specific infectious diseases, according to Dr. Anneli Lauhio, a specialist in infectious diseases in Finland who has studied the morbidity caused by these illnesses. Doing so through large-scale studies, she said, “will change medicine a lot,” and help doctors identify these complications earlier and treat them.
We are starting to see strides in this direction. The U.S. National Institutes of Health has allocated $1.15 billion in funding for research into the prolonged health consequences of SARS-CoV-2 infection. Part of this effort will include large studies involving electronic health records, to capture a broad amount of data over time. Similarly, researchers in England and Australia created a global registry that will collect information about cases of newly diagnosed diabetes following Covid-19; some have suggested the infection elevates diabetes risk.