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Your body is teeming with bugs. There are at least as many bacteria in us as there are human cells. It’s estimated we contain 100 times more viruses than bacteria. Luckily, the vast majority of these entities cause us no harm. Many even improve our health! But of among this diverse set of microbes, some cause diseases like pneumonia, skin infections, and hepatitis. Within this set of bugs (called pathogens), a few seem to directly hack into our brains, changing what we do and how we think.
Let’s be clear: just about any significant infection can alter our thinking and our actions. A bad flu, for example, leads us to stay home from work and drink chicken soup. It can also cause foggy thinking, and in some cases, even hallucinations. Any condition that causes significant inflammation of the brain can lead to confusion, agitation of other alterations in consciousness. It’s also thought that the vast array of bugs in the gut (called the microbiome) may influence our thinking. But of all of these, three specific microbes seem to have particularly notable effects on our thoughts and actions.
1. Rabies
Thousands of years ago, Hippocrates described a condition where “persons in a frenzy drink very little, are disturbed and frightened, tremble at the least noise, or are seized with convulsions.” While less of an issue in the developed world, rabies remains one of the most feared diseases, with a fatality rate of nearly 100 percent once symptoms occur. But it’s not just death that makes this illness so terrifying. The virus has a tendency to infect the brain, where it appears to alter human behavior.
When the rabies infection enters the body, it searches for nerve cells. Once found, the virus ascends through the cell body until it reaches the brain. From there it spreads outward to peripheral and autonomic nerves throughout the body. At the end of this process, it moves into the salivary glands, where it can be transmitted to the next host through a bite.
In animals, especially dogs, rabies is linked to aggressive behavior and increased biting. In approximately 80 percent of human cases of rabies (furious rabies), hyperactivity and agitation can be seen. There can also be more unique symptoms. These include fear of water (hydrophobia) and fear of puffs of air (aerophobia).
2. Toxoplasmosis
Toxoplasma gondii (toxo) was first described in the scientific literature around a century ago, and then isolated as a threat to humans a few decades later. While this parasite only reproduces in cats, it can still infect and cause significant issues in humans. This bug remains a major global threat, with up to 50 percent of humans currently infected.
Even among the brain-warping bugs, toxo plays dirty. It’s reported that the parasite warps the brains of mice so that they become sexually attracted to cat odors. This may prompt them to seek out felines, which gets them eaten, and gets the parasite where it needs to reproduce.
In humans, toxo has been linked to a range of behavioral changes. For example, toxo antibodies are associated with an increased risk of traffic accidents and suicide attempts. It’s additionally a risk factor for OCD, antisocial personality disorder, schizophrenia, and ADHD. While the exact mechanisms remain unclear, there’s evidence that the bug may alter levels of the neurotransmitter dopamine in the brain.
3. SARS-CoV-2
The spread of the 2019 coronavirus (SARS-CoV-2) across the globe has had effects on multiple aspects of human behavior. While many may not be completely unique to this infection, the reach of COVID-19 is unprecedented. Among the neurological effects of this virus are increasingly recognized effects on our cognition and psychology. A number of recent studies have focused on the brain-related effects of viral infection, but the indirect impact of the virus on our collective thinking is an even bigger effect.
Over the course of the last year, researchers have investigated whether COVID-19 can influence our thinking. Much of the data has focused on how the virus may affect our central nervous system, and what that might look like from the outside. For example, some have found that COVID-19 infection may confer a risk for cognitive impairment on some tests of brain function. The exact reason for these symptoms is still being investigated, but it may be by way of brain inflammation. For example, COVID-19 is thought to activate microglia (the brain’s immune cells), which are involved in essential activities for cognition including neuroplasticity and memory.
Other research focuses more explicitly on the psychological impact of infection and has alluded to an increased risk for anxiety and depression months after the initial disease onset. It’s also notable that the global economic and social mental health burden imposed by virus-related policy changes are likely to have a detrimental effect on worldwide mental health.
When it comes to the indirect effects of COVID-19 on our thinking, the fallout from increased stress is an essential consideration. Stress, especially at high and sustained levels, is known to be damaging to most aspects of our physiology. In some people, chronic stress may lead to more habitual behavior and impairments in behavioral flexibility. Animal data also suggest that chronic stress may help facilitate addiction-related brain circuits. In humans, self-reported stress correlates with more impulsive decision-making. In total, these data suggest that stress related to the COVID-19 pandemic may lead to worse decisions, especially if our pre-existing habits were unhealthy.
