Doctors around the world are trying to make sense of the wide range of effects that COVID-19 the disease caused by the novel coronavirus can have on the body.

Most people associate the infection with respiratory symptoms fever, cough, fatigue. But many patients who contract the virus experience neurological symptoms such as headache and dizziness, as well as heart issues, kidney complications and, most recently, blood clotting (also called coagulation).

The clots have taken various shapes and forms in different patients. In milder cases, the clots which appear to be scattered throughout the body have been linked to rashes or swollen, red toes (recently deemed COVID toes). In more severe cases, the clots can block an artery and cause a pulmonary embolism or trigger a heart attack or stroke.

Its unclear why so many patients admitted to the hospital with COVID-19 are developing clots. One study found that nearly a third of COVID-19 patients in intensive care had clotting.

Weve seen clotting with other severe respiratory infections before, so doctors have a few theories as to whats going on. Heres what we know about COVID-19 and blood clotting.

The Clots Are Often Tiny And Everywhere

One of the bizarre findings about the clotting observed in coronavirus patients is how minuscule and widespread the clots are.

When we typically talk about blood clots, were looking at one or two major clots that block off blood supply to the brain and heart (and therefore cause a stroke or heart attack). But with COVID-19, rather than one or two big clots, doctors are seeing tons of tiny clots forming all over the place in the lungs, mostly, but also in the legs, feet and just below the surface of the skin (which may be causing those COVID toes).

Weve seen big clots in legs and big clots in the large vessels of the lungs as well [with COVID-19], but it seems like with COVID-19 patients, you have this additional micro-thrombotic [small clotting] effect going on, Matthew Heinz , a hospitalist and internist at the Tucson Medical Center, told HuffPost.

You may think itty bitty clots would be less harmful. But Heinz said they may actually be an indication that a much more severe illness is brewing. Just because the clots tend to be smaller and evenly distributed doesnt mean that they cant be life-threatening.

They can indicate that clotting cascade those clotting derangements has now begun, and they could start to form everywhere, and you could end up with little clots forming in the coronary arteries that would give you a heart attack. And in the small arteries of the brain, that could give you focal strokes, Heinz said.

This is probably why were seeing coronavirus patients in their 30s and 40s who dont have any other risk factors develop strokes, Heinz added.

Other Coronaviruses Have Been Linked To Clotting Issues

Hyung Chun , a Yale Medicine cardiologist, said other coronaviruses known to cause severe respiratory disease (such as MERS and SARS) are also associated with blood clots.

The SARS coronavirus damages the inner lining of the arteries and veins (known as the vascular endothelium ), which prevents blood from reaching the lungs, triggering a pulmonary infarction in which lung tissue essentially dies in some patients. Another analysis of SARS found that about 49% of patients developed thrombocytosis , a condition that causes blood clots to spontaneously appear.

With MERS , it was estimated that about 36% of patients developed a coagulation or blood clot disorder. Additionally, MERS patients who got the sickest often experienced a complication called disseminated intravascular coagulation , in which small blood clots form throughout the bloodstream and block the flow of small blood vessels.

Given the similarity between the viruses that cause SARS, MERS and COVID-19, researchers expect to see similar blood clotting complications . (Side note: we also see clotting with severe influenza and viral pneumonias , according to Chun, but that seems to be rarer.)