The patient had been relatively fine for the first 10 days he was down with Covid-19.

Just 38, he didn't fit the description of people at high risk of complications from the new coronavirus.

"He had mild pulmonary symptoms that he was just sitting at home with," said Dr. Sean Wengerter, a vascular surgeon in Pomona, New York. "He had been diagnosed at an urgent care clinic and it was going fine at home. He just had a little cough."

Until one of Covid-19's surprising effects kicked in.

"Then he just woke up with both his legs numb and cold and so weak he couldn't walk," said Wengerter, who is division chief of vascular surgery at Westchester Medical Center Health's Good Samaritan Hospital.

Coronavirus can cause blood clots

This relatively young man had an aortic occlusion -- a big blood clot in the body's main artery, right above where it splits into two parts to run into each leg. Blood was not getting into the iliac arteries and his legs were being starved.

It's an extremely dangerous development that can kill between 20% and 50% of patients, Wengerter said. "It just doesn't usually happen in a 38-year-old," he told CNN.

Quick diagnosis and a surgical procedure to slice open the arteries and scoop out the clot using a catheter saved the patient. "We had two surgeons working simultaneously on him," Wengerter said.

Doctors treating coronavirus patients are seeing a range of odd and frightening syndromes, including blood clots of all sizes throughout the body, kidney failure, heart inflammation and immune complications.

"One thing that is both curious and evolving and frustrating is that this disease is manifesting itself in so many different ways," said Dr. Scott Brakenridge, an assistant professor on the acute care surgery team at the University of Florida College of Medicine.

It can also cause multi-system organ failure

"In some cases it's having severe effects on the patient's ability to breathe, and in others it seems to be associated with development of multi-system organ failure -- when all your organs shut down. And now it's associated with immune effects in children."

While the new coronavirus is designated as a respiratory virus, it's clear that it is affecting some people throughout their bodies. The most obvious symptoms of infection are classic respiratory symptoms: fever, pneumonia and acute respiratory distress syndrome.

But the virus also seems to attack some organs directly. One of the most troubling is its assault on the lining of the blood vessels, which in turn causes unnatural blood clotting.

"It seems like Covid, the virus, is creating a local inflammatory response that's leading to some of these thrombotic events," Wengerter said. "This is happening because of the direct action of the virus on the arteries themselves."

Other teams of doctors have reported unusual strokes in younger patients, as well as pulmonary embolisms, the medical name for blood clots in the lungs.

Pathologists are finding tiny blood clots in the smallest vessels, as well, said Dr. Oren Friedman, who has been taking care of Covid-19 patients in the intensive care unit at Cedars-Sinai Medical Center in Los Angeles.

"There's no debate -- the virus seems to affect thrombosis and seems to directly affect your blood vessels," Friedman told CNN. And that means it affects the whole body.

"Obviously, every single organ in your body is fed by blood vessels, so if the virus affects your blood vessels, then you can have organ damage," he said.

"It is a very confusing picture. It's going to take time to understand," Brakenridge said.

It might cause children's immune systems to overreact

One of the most frightening syndromes that might be linked with Covid-19 is "pediatric multisystem inflammatory syndrome." New York City reports 52 cases, Mayor Bill de Blasio said Tuesday, and the New York State Department of Health says it is investigating 100 cases.

It is characterised by persistent fever, inflammation, poor function in one or more organs, and other symptoms that resemble shock, a panel of paediatricians known as the International PICU-COVID-19 Collaboration says.

"In some cases, children present with shock and some have features of Kawasaki disease, whereas others may present with signs of cytokine storm. In some geographic areas, there has been an uptick in Kawasaki disease cases in children who don't have shock," Boston Children's Hospital rheumatologist Dr. Mary Beth Son said. Kawasaki disease involves inflammation in the walls of medium-sized arteries and can damage the heart.

It may be caused by an immune system response known as a cytokine storm, doctors say.

"Your immune system is overreacting to the virus, and because these are inflammatory diseases, this overreaction can cause a Kawasaki-like disease," Dr. Glenn Budnick, a pediatrician in Pomona, New Jersey, said on CNN Newsroom Saturday.

"It is even possible that the antibodies that children are making to SARS-CoV2 are creating an immune reaction in the body. Nobody knows," said Dr. Jane Newburger, a cardiologist on the Boston Children's panel and an expert on Kawasaki disease.

Cytokine storm may also cause some of the lung damage and unusual blood clotting seen in adult patients, doctors said.

"There is other evidence that the virus really doesn't generate a strong immune response and actually it is suppressing the immune system," Brakenridge said. That would allow the virus to more directly attack organs.

A study published in the journal Nature Medicine on Tuesday supported both theories.

Dr. Zheng Zhang and colleagues at Shenzhen Third People's Hospital in Shenzhen, China analyzed samples of immune cells taken from the lungs of nine coronavirus patients and found abnormally high levels of immune cells called macrophages and neutrophils, as well as immune signaling chemicals called cytokines and chemokines in the sicker patients. Sicker patients also had high levels of proliferating T-cells, another type of immune cell.

But the patients with the most severe symptoms had lower numbers of CD8 T-cells, which directly kill virus-infected cells.

Doctors say they are finding that various treatments can help control symptoms. Blood thinners can help control the unusual blood clotting, while immune blockers may help control the cytokine storm.

It can cause 'Covid toes'

One last symptom that is puzzling -- but less troubling -- is known as "Covid toes." Patients are reporting red or purple swelling of their toes.

It's possible the tiny blood clots associated with Covid-19 are causing it, doctors said.

"One pattern of COVID toes that people are reporting is red lesions typically on the soles. It's possible that this is a skin reaction or caused by a small clog or micro clots in the blood vessels found in the toes," Cleveland Clinic pulmonologist Dr. Humberto Choi said on the clinic's website.

It's not usually associated with any serious symptoms, Choi said.