(CNN) It may feel like years, but the world has only been battling the deadly novel coronavirus for months. As scientists feverishly work around the clock to unravel the mysteries of this deadly invader, they have gained interesting insights into the virus and the disease it causes: Covid-19.

"We know more now about how it makes people sick," said infectious disease expert Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville.

Fever, cough and shortness of breath are still key signs that you might have Covid-19, but experts now know there are other red flags signaling infection. Some mimic a typical cold; others mirror the flu; and a few are just downright bizarre.

"We've learned about Covid toes, the loss of taste and smell," Schaffner said."We've learned that it can actually produce delirium — we're not entirely sure how it affects the central nervous system yet, but it's clear that some of these people have delirium."

Any or all symptoms can appear between two to 14 days after exposure to the virus, according to the US Centers for Disease Control and Prevention . But many doctors find most people will show signs of the disease within a week,Scaffner said.

"We think the average incubation period is shorter," he said. "It's five to seven days now, and occasionally it can go longer."

If there are any symptoms at all. That's yet another thing science has learned: Many of us can be asymptomatic, or at least think we are, and spread the virus without knowing it.

In fact, Schaffner said, most studies are finding that a third or more of people who test positive are without symptoms or have very mild symptoms they may not even notice.

"You and I feel perfectly well, and we could be sitting opposite each other at a table having this discussion, but I can't prove to you that I don't have the virus. And you can't prove to me that you don't," Schaffner said.

"That's the whole rationale for mask wearing by everybody," he added. "And that's another thing we've learned — masks really do offer protection for the wearer as well as others. That's important for people to understand, because a lot of people have said, 'Well, if I'm not sick, I don't need to wear this mask.' And that's unwise."

Read more: Small actions you can take to prevent coronavirus spread

Unusual symptoms

If there was ever a disease that has figured out a way to tackle the body in weird and widespread ways, Covid-19 is it. It appears to be able to invade nearly every major organ. It clots blood, often in dangerous ways. It attacks our sense of smell, our eyes, even our skin.

Because new, rarer symptoms continue to pop up as more and more of us experience Covid-19, this is not an exhaustive list. Still, here are a few of the bizarre symptoms we might experience.

A new loss of taste or smell: The CDC recently added this unusual symptom to its list of top signs that you might have Covid-19. It can occur without any prior warning, not even a stuffy nose.

"What's called anosmia, which basically means loss of smell, seems to be a symptom that a number of patients developed," CNN Chief Medical Correspondent Dr. Sanjay Gupta told CNN anchor Alisyn Camerota on " New Day ."

It appears to be more prevalent in mild or moderate cases of Covid-19, and tends to appear at the beginning of the illness. It may be even be one of the first signs that you are sick.

"Anosmia, in particular, has been seen in patients ultimately testing positive for the coronavirus with no other symptoms," according to the American Academy of Otolaryngology-Head and Neck Surgery.

Any respiratory virus, such as cold or flu, will temporarily impact smell and taste, and in rare cases, the loss can be permanent. So the loss of those senses are not a definitive diagnosis of Covid-19.

Still, "it's clearly something to look out for — sometimes these early symptoms aren't the classic ones," Gupta said.

Is there anything you can do at home to test to see if you're suffering a loss of smell? The answer is yes, by using the "jellybean test" to tell if odors flow from the back of your mouth up through your nasopharynx and into your nasal cavity. If you can pick out distinct flavors such as orange and lemon, your sense of smell is functioning fine.

Covid toes and blood clots: At first glance, the swollen red or purple toes look like a case of frostbite. But it's just another example of the strange ways that Covid-19 — primarily a respiratory disease — affects the body.

What's really going on? The virus is creating tiny blood clots in the smallest blood vessels in the feet. And while colorful toes may not be a cause for alarm, experts say, the discovery that Covid-19 is clotting blood throughout the body is a dangerous development.

Doctors are finding blood clots of all sizes throughout the body, even in people who are young and healthy. Those clots are often lodging in the limbs of the body, where they can break away and clog the lungs, heart, kidneys and liver. There the clots cause inflammation, immune complications and even deadly or debilitating strokes and pulmonary embolisms, the medical name for blood clots in the lungs.

"It seems like Covid, the virus, is creating a local inflammatory response that's leading to some of these thrombotic events," said Dr. Sean Wengerter, division chief of vascular surgery at Westchester Medical Center Health's Good Samaritan Hospital, in a prior CNN interview. "This is happening because of the direct action of the virus on the arteries themselves."

Signs of a potential clot in a limb include unusual tenderness or pain, a red or blue tinge to the area, warmth or itchiness or cramping in the lower calf or leg. The clot may have moved to lungs or heart if you have chest pain, dizziness, a bad cough or trouble breathing.

Pink eye: Early research from China, South Korea and other parts of the world found about 1% to 3% of people with Covid-19 also had conjunctivitis, commonly known as pink eye.

Conjunctivitis, a highly contagious condition when caused by a virus, is an inflammation of the thin, transparent layer of tissue, called conjunctiva, that covers the white part of the eye and the inside of the eyelid.

But SARS-CoV-2 is just one of many viruses that can cause conjunctivitis, so it came as no real surprise to scientists that this newly discovered virus would do the same.

Still, a pink or red eye could be one more sign that you should call your doctor if you also have other telltale symptoms of Covid-19, such as fever, cough or shortness of breath.

Sudden confusion, even delirium: The World Health Organization lists altered consciousness or confusion as a possible early symptom of Covid-19, in some cases presenting even before fever and cough. Delirium and hallucinations can appear, which can have long-term impact — slowing recovery and increasing the risk for dementia, depression or post-traumatic stress.

And it's not just the mentally weaker elderly who are suffering. Doctors see many younger patients with such symptoms as well. Studies show some 60% to 75% of Covid-19 patients in intensive care are experiencing various central nervous system breakdowns: Some hallucinate they are on fire or freezing; others see visions they keep to themselves, becoming withdrawn; and still others are confused and irritable.

Because signs of encephalopathy — any damage or disease that affects the brain — can lead to serious illness, the CDC says that any sudden confusion or an inability to wake up and be alert is a serious symptom.

If you or a loved one has those symptoms, especially with other critical signs like bluish lips, trouble breathing or chest pain, the CDC says to seek help immediately by calling 911.

10 typical symptoms

Some of the more well-known and commons signs of Covid-19 can be just as dangerous.

An inability to take a deep breath: Shortness of breath is not usually an early symptom of Covid-19, but it is the most serious. It can occur on its own, without a cough. If your chest becomes tight or you begin to feel as if you cannot breathe deeply enough to fill your lungs with air, that's a sign to immediately call your doctor or a local urgent care, experts say.

"If the shortness of breath is severe enough, you should call 911," said American Medical Association president Dr. Patrice Harris.

Get medical attention immediately, the CDC says, if you experience a "persistent pain or pressure in the chest," or have "bluish lips or face," a possible sign of a lack of oxygen.

A rising temperature: Fever is a key sign of Covid-19. But don't fixate on a number on the thermometer. Many people have a core body temperature that is above or below the typical 98.6 degrees Fahrenheit (37 degrees Celsius).

CNN anchor Chris Cuomo, who battled the virus in late March and early April from his home in New York, is one of those people.

"I run a little cool. My normal temperature is 97.6, not 98.6. So, even when I'm at 99 that would not be a big deal for most people. But, for me, I'm already warm," Cuomo told CNN Chief Medical Correspondent Dr. Sanjay Gupta in a CNN Town Hall.

Most children and adults, however, will not be considered feverish until their temperature reaches 100 degrees Fahrenheit (37.7 degrees Celsius).

"There are many misconceptions about fever," said Dr. John Williams, chief of the division of pediatric infectious diseases at the University of Pittsburgh Medical Center Children's Hospital of Pittsburgh.

"We all actually go up and down quite a bit during the day as much as half of a degree or a degree," Williams said, adding that for most people "99.0 degrees or 99.5 degrees Fahrenheit is not a fever."

Don't rely on a temperature taken in the morning, Schaffner advised, because temperature varies during the day. Instead, take your temperature in the late afternoon and early evening.

"One of the most common presentations of fever is that your temperature goes up in the late afternoon and early evening. It's a common way that viruses produce fever."

A debilitating cough: Coughing is another common symptom, but it's not just any cough, Schaffner said. The cough is bothersome, one that you feel deep in your chest.

"It's not a tickle in your throat. You're not just clearing your throat. It's not just irritated," Schaffner explained. "It's coming from your breastbone or sternum, and you can tell that your bronchial tubes are inflamed or irritated."

Around 60% of the people infected with Covid-19 experienced a dry, peristent cough, likely from irritated lung tissue, according to studies from China . As the disease worsens, the lungs begin to fill with fluid, which might change the nature of the cough.

report put out by the World Health Organization in February found over 33% of 55,924 people with laboratory-confirmed cases of Covid-19 coughed up sputum, a thick mucus sometimes called phlegm, from their lungs, which is also characteristic of the common cold or flu.

Chills and body aches: "The beast comes out at night," said Cuomo, referencing the chills, body aches and high fever that visited him on April 1.

as quarantined from the rest of his family until April 20. 'It was like somebody was beating me like a piñata. And I was shivering so much that ... I chipped my tooth. They call them the rigors, " he sa id from his basement, where he ws quarantined from the rest of his famiy until April 20.

"I was hallucinating. My dad was talking to me. I was seeing people from college, people I haven't seen in forever, it was freaky," Cuomo said.

Not everyone will have such a severe reaction, experts say. Some may have no chills or body aches at all. Others may experience milder flu-like chills, fatigue and achy joints and muscles.

Of course that makes it difficult to know if you have the flu or have been exposed to the novel coronavirus. One possible sign that you might have Covid-19 is if your symptoms don't improve after a week or so but actually worsen.

Overwhelming exhaustion: For some people, extreme fatigue can be an early sign of the novel coronavirus. The WHO For some people, extreme fatigue can be an early sign of the novel coronavirus. The WHO report found nearly 40% of the nearly 6,000 people with laboratory-confirmed cases experienced fatigue.

Just a few days into his quarantine, Cuomo was already drained by the fevers and body aches the disease brings.

"I'm so lethargic that I can stare outside, and, like, an hour-and-a-half goes by," Cuomo told Gupta on Anderson Cooper 360 . "I think I took a 10-minute nap, and it was three and a half hours."

Fatigue may continue long after the virus is gone. Anecdotal reports from people who have recovered from Covid-19 have said exhaustion and lack of energy continue well past the standard recovery period of a few weeks. Some continue to experience fatigue and other symptoms for months. As more cases of lengthy illness appear, these unlucky victims of the virus are now being described as "long-timers."

Diarrhea and nausea: At first science didn't think gastric issues that often come with the flu applied to Covid-19. After all, this is a respiratory disease. But as more research rolled in and the At first science didn't think gastric issues that often come with the flu applied to Covid-19. After all, this is a respiratory disease. But as more research rolled in and the virus was found in stool samples , that opinion quickly changed.

"In a study out of China where they looked at some of the earliest patients, some 200 patients, they found that digestive or stomach GI (gastrointestinal) symptoms were actually there in about half the patients," Gupta said on CNN's "New Day" news program.

Researchers have now found that SARS-CoV-2 can infect cells in the intestine and multiply there. That's likely because, like the lungs, the intestinal lining is loaded with ACE2 receptors, the sweet spot where the spokes of the SARS-CoV-2 virus attach and enter the body's cells.

Sore throat, headache and nasal congestion: The early WHO report also found nearly 14% of the almost 6,000 cases of Covid-19 in China had symptoms of headache and sore throat, while almost 5% had nasal congestion.

Today, science recognizes those very common signs of both the common cold and flu as more frequent in Covid-19 than previously thought. The CDC added both sore throat and headache to its most recent update of common Covid-19 symptoms on the recommendation of the Council of State and Territorial Epidemiologists, who advise the agency.

While cough, shortness of breath or difficulty breathing are key red flags of Covid-19, the council recommended headache and sore throat fall into a second tier of symptoms that may trigger doctors to consider testing for Covid-19.

That tier kicks in when patients present with two of the following clinical signs: Chills or shivers, fever, muscle pain, headache, sore throat or new loss of taste and smell. If there is no other more appropriate diagnosis, the guidelines say doctors should report those patients as potentially infected with Covid-19.

Who is most at risk?

Everyone — every man, woman and child in the world — is equally at risk of being infected with Covid-19. How the disease will then impact you or your loved one depends on many variables, including some that science does not yet understand.

Why do some people — even the elderly — have few or no obvious symptoms? Why do others — despite being young and healthy — quickly collapse, go into a cytokine storm and die? Why do a rare subset of children who have recovered from Covid-19 present weeks later with a deadly toxic shock-like illness called "pediatric multisystem inflammatory syndrome"?

No one yet knows.

Age appears to play a role — most children and young adults appear to weather the virus more successfully, but not all. Older adults appear to be the most fragile, but that's likely due to their weaker immune systems and chronic conditions.

One thing is very clear: The highest risk of severe illness and death is for anyone with an underlying health condition. The list includes diabetes, chronic lung disease or asthma, cardiovascular disease, cancer (or are undergoing chemotherapy), organ transplants, sickle cell anemia, kidney disease with dialysis, poorly controlled HIV infection, any autoimmune disorder or a body mass index (BMI) over 30 (obese).

To put that in context: More than 40% of the American population have a BMI of over 30 and are considered obese, while an estimated 60% of American adults have at least one chronic medical condition, according to the CDC.

"Older patients and individuals who have underlying medical conditions or are immunocompromised should contact their physician early in the course of even mild illness," the CDC advises.

To be clear, you are at higher risk — even if you are young — if you have underlying health issues.

Pregnancy is also a risky time, science has discovered. Early in the pandemic, the CDC didn't think Covid-19 would raise the risk for either expecting mother, fetus or newborn.

But the agency added pregnancy to its list of top risk factors, after a study found pregnant women who get infected are more likely to be hospitalized, admitted to an intensive care unit and put on a ventilator.

"Pregnant women were 50% more likely to be admitted to the intensive care unit and 70% more likely to receive mechanical ventilation," said Dr. Sara Oliver of the CDC's National Center for Immunization and Respiratory Diseases in late June

How to be evaluated

Think you might have signs of the novel coronavirus? With the explosion of cases this summer across the United States, many hospitals are again bursting at the seams. Unless you are experiencing emergency signs of Covid-19, which include trouble breathing, persistent chest pain or pressure, new confusion, an inability to wake or stay awake or a bluish tinge to lips or face, you should call your health provider first. If you do have any alarming symptoms, call 911.

Get CNN Health's weekly newsletter Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

"If you have insurance and you're looking for a provider or someone to call or connect with, there's always a number on the back of your insurance card; or if you go online, there is information for patients," said the AMA's Harris.

"If you don't have insurance, you can start with the state health department or the local community health centers, those are officially known as federally qualified health centers," Harris advised, adding that some states have a 1-800 hotline number to call.

"If there is a testing and assessment center near you, you can go there directly," said Vanderbilt's Schaffner. "It's always good to notify them that you're coming. Otherwise, you need to call your healthcare provider and they will direct you what to do."