Exactly what it is, when it appears, how long it lingers, how many patients it affects (from 10% to 80% of COVID survivors, depending on the study), and what just causes it—beyond the seemingly obvious—are debated, with new hypotheses floated daily.

As research into the vexing new condition expands, it seems that almost anything and everything could be a symptom—from ear numbness, a sensation of “brain on fire,” and erectile dysfunction to irregular menstrual periods, constipation, and peeling skin.

For those investigating the mysterious malady, that’s a real problem.

Recently scientists have attempted to categorize long COVID patients into subgroups, hypothesizing that the condition federal officials say could affect up to 23 million Americans isn’t one thing but many.

A picture is beginning to emerge of “groups of symptoms that have yet to be characterized well enough to call one or several diseases,” Justin Reese, a computational biologist with Berkeley Lab—a multiscience lab supported by the U.S. Department of Energy—told Fortune this week via email. Reese recently lead a study that used machine learning to classify Long COVID patients.

Another study released last month used artificial intelligence to classify tens of thousands of U.S. “long-haul” patients using their electronic health records. The main symptom sets found include patients primarily suffering from:

• Blood and circulatory system issues like heart failure, arrhythmias, and chest pain.

• Respiratory system issues like throat and chest pain, upper respiratory infections, asthma, and lower respiratory disease.

• Musculoskeletal and nervous system issues like connective tissue disorders, osteoarthritis, and musculoskeletal pain.

• Digestive system issues like abdominal and pelvic pain, nausea, vomiting, and disorders of the urinary system.

But theories abound. Dr. Alexandra Brugler Yonts, an infectious disease specialist at Children’s National Hospital in Washington, D.C., recently shared her own list of possible buckets with Fortune. They include patients who:

• Have sustained long-term organ injury from COVID, such as lung damage.
  
• Are suffering from inflammation resulting from COVID.
  
• Have developed dysautonomia, which leads to changes in blood pressure and heart rate upon changes in position, like sitting or standing.
  
• Have ongoing COVID viral activity.
  
• Have immune systems that overcorrected in response to COVID and now under-respond to pathogens.

Categories may extend well beyond a handful, however, as symptoms have been identified in 10 of the 11 organ systems, according to a landmark study published in British medical journal The Lancet in July.

Here are 100 of the most common symptoms the study identified, by category:

Emotion and mood

• Anxiety
• Irritability
• Depression
• Tearfulness
• Apathy
• Sense of doom
• Depersonalization
• Anger
• Impulsivity
• Suicidality
• Euphoria
• Delusions

Sensorimotor

• Dizziness
• Pins and needles feeling
• Tremors
• Numbness
• Noise sensitivity
• Ringing in the ears
• Feeling of brain pressure
• Nerve pain
• Sensation of brain “on fire”
• Inability to yawn

Sleep

• Insomnia
• Awakened by feeling of inability to breathe
• Vivid dreams
• Nightmares
• Restless leg syndrome
• Sleep apnea

Cognitive functioning

• Brain fog
• Poor attention span
• Difficulty thinking
• Sudden confusion
• Racing thoughts

Memory

• Short-term memory loss
• Long-term memory loss
• Forgetting how to do routine tasks
• Inability to make new memories

Headaches

• Behind the eyes
• All over
• Temples only
• Base of skull
• After mental exhaustion
• Migraines

Taste and smell

• Loss of taste
• Loss of smell
• Altered taste
• Altered smell
• Heightened taste
• Heightened smell
• Phantom smells
• Phantom taste

Speech and language

• Difficulty finding the right words
• Difficulty reading
• Difficulty understanding speech
• Difficulty speaking
• Difficulty writing
• Slurred speech
• Speaking unrecognizable words

Hallucinations

• Visual
• Auditory
• Tactile

Systemic

• Chills, sweats
• Elevated temperature
• Fatigue
• Low temperature

Pulmonary, respiratory

• Breathing difficulties but normal oxygen saturation
• Cough with mucus
• Coughing up blood
• Dry cough
• Sinus issues
• Rattling of breath
• Shortness of breathe
• Sneezing

Reproductive, genitourinary, endocrine

• Menstrual issues
• Bladder control issues

Cardiovascular

• Fainting
• Heart palpitations
• Burning in the chest
• Bulging veins

Dermatologic

• “COVID toe”
• Peeling skin
• Rashes

Gastrointestinal

• Abdominal pain
• Constipation
• Diarrhea
• Loss of appetite
• Reflux
• Nausea
• Vomiting

Immunological, autoimmune

• New allergies
• New anaphylaxis reactions

Musculoskeletal

• Bone aches
• Joint pain
• Muscle aches
• Muscle spasms
• Chest tightness

Head, ear, eye, nose, throat

• Hearing loss
• Runny nose
• Sore throat
• Vision symptoms