How Do Infections Affect the Brain?

Brain infections exist, but infection does not have to start in the brain to impact it. Many viral, fungal, and bacterial infections start elsewhere in the body and can spread to or in some way affect the brain. How the brain is impacted varies depending on the type of infection.

Types of Infections

How infections impact the brain and the rest of the body depends on the type of infection. Infections are caused by bacteria, viruses, fungi, or parasites, and the source of the infection can affect the symptoms.For example, a viral or bacterial infection can cause a fever, but in bacterial infections, fevers are likely to reach much higher temperatures.

HIV and AIDS

HIV is the virus that causes AIDS. It attacks the immune system, leaving the person vulnerable to infection. But HIV also directly affects the brain, infecting the cells that protect and support neurons and triggering inflammation.These changes can cause confusion, forgetfulness, mood disorders, headaches, and movement problems. Some research also shows that HIV can shrink certain brain structures and impact information processing.

Strep A

Group A strep causes a range of infections, from strep throat to rheumatic fever to toxic shock syndrome. Studies show that recurrent strep infections can affect the brain, especially in children.Strep A has molecules in its bacterial cell wall similar to those found in brain tissue. The immune system recognizes these molecules, and instead of attacking them, mistakenly attacks the body's tissue instead. This leads to an autoimmune response and inflammation, increased blood-brain permeability, and impairments in brain function.

Herpes Simplex 1

Herpes simplex 1 (HSV-1) is a virus that causes cold sores and genital herpes. Rarely, after the initial infection, the virus reaches the central nervous system and replicates, causing inflammation and leading to encephalitis.Interestingly, HSV-1 encephalitis has been linked with the accumulation of certain molecules that are considered biomarkers of Alzheimer's disease.

Cytomegalovirus

Cytomegalovirus or CMV is in the same family of viruses that cause cold sores and chickenpox. Most people with a CMV infection have no or mild symptoms, but people with a compromised immune system may have a more severe infection that affects the nervous system, including the brain.It can cause inflammation in the brain and spinal cord, and unborn infants whose mothers have CMV infections have an increased risk of hearing loss, seizures, and visual impairments at birth.

Rabies

Unlike most infections, rabies travels directly from the point of entry — the site of the infected animal bite — to the brain. This process can take as long as ten days, depending on the location of the bite. Once it reaches the brain, the virus begins to multiply, and the disease is usually fatal.Early symptoms are headache and a fever, but once the infection reaches the brain, symptoms include hallucinations, confusion, excitement. As the infection spreads through the brain, the person may experience seizures, paralysis, coma, or death.

Meningitis

Meningitis is an infection of the lining of the spinal cord and brain. This infection causes these membranes to swell, pressing against the brain and spinal cord, which causes life-threatening complications. Symptoms, like headaches, high fevers, and confusion, often come on suddenly and worsen quickly.Many bacteria and viruses can cause meningitis. Viral meningitis is more common, but bacterial meningitis is more severe and can lead to stroke, paralysis, or brain damage. When it is caught and treated early, many people recover from meningitis without any permanent damage. But even with prompt and effective treatment, this infection can result in brain damage, hearing loss, seizures, and permanent disabilities.

Brain Abscesses

Infections of the middle ear, sinuses, or the mastoid cells in the temporal bones of the face can spread to the brain and cause a brain abscess. Brain trauma or brain surgery can also cause these growths.A brain abscess is a pocket filled with infected material or pus. It can cause swelling or cut off blood flow to parts of the brain and is a medical emergency. Signs of a brain abscess include fever, nausea, vomiting, confusion, weakness on one side of the body, and pain in the neck or back.

COVID-19

COVID-19 has a wide range of symptoms that affect various parts of the body, including the brain. Studies show that 80 percent of people hospitalized with COVID-19 had some kind of neurological symptoms, including loss of taste and smell, brain damage, difficulty remembering words, coma, and stroke.Researchers also found that people with pre-existing neurological conditions, like nerve diseases, dementia, and chronic migraines, were more likely to experience neurological symptoms with COVID-19.

Infection and the Blood-Brain Barrier

The blood-brain barrier is the barrier between brain's blood vessels and the brain, made up of a dense network of cells. It has special properties that tightly regulate the movement of cells, molecules, and ions in and out of the brain to maintain balance and keep out toxins, disease, and pathogens.Systemic infection and inflammation can disrupt this barrier, making it more permeable. While this may explain why so many infections impact the brain, researchers are also interested in whether they can use this permeability therapeutically, indicating hope for future treatment of many conditions.