What exactly are 'underlying conditions?' And why people with them may experience more serious illness from coronavirus

(CNN)We've heard that elderly people and those with underlying health conditions are most at risk if they're infected with coronavirus, but those can seem like really general terms. Who does that include? And why can they face more serious illness?

"According to the [Centers for Disease Control and Prevention], some of the underlying conditions that may put you at higher risk include: chronic lung disease and asthma, heart disease and undergoing cancer treatment," said CNN Chief Medical Correspondent Dr. Sanjay Gupta in an episode of CNN's "Coronavirus: Fact vs. Fiction" podcast. Anyone with diabetes, kidney failure or liver failure may also be at higher risk.
The role of the immune system is to protect against disease or other potentially damaging pathogens. A strong one is needed to help stave off coronavirus infection.
    "Think of it like this," Dr. Gupta suggested. "In your everyday life, you're always fighting off pathogens. Most of the time you don't even realize it. If you have an underlying condition, it makes it more challenging to fight off a virus like this. You may develop a fever, shortness of breath or a cough more easily than someone who doesn't have a preexisting illness."
      Additionally, there are more specific reasons why each condition has its own vulnerabilities. Here's a guide to underlying conditions affected by coronavirus and why, and how you can protect yourself or an at-risk loved one.

      Older adults

      Eight out of 10 deaths reported in the US have been in adults ages 65 and older, according to the CDC. Older adults have also been more likely to require hospitalization and admission to an intensive care unit.
        Older adults are more likely to have long-term health problems that can increase their risk for infection and serious disease. And, our immune systems usually weaken with age, making it more difficult for people to fight off infections, according to Johns Hopkins Medicine.
        The quality of our lung tissue also declines over time, becoming more elastic and making respiratory diseases such as Covid-19 of important concern because of the potential for lung damage.
        Inflammation in older adults can be more intense, leading to organ damage.

        Those with lung disease, asthma or heart conditions

        People with chronic airway and lung diseases such as chronic obstructive pulmonary disease (COPD), asthma, pulmonary fibrosis and interstitial lung disease can lay the foundations for more severe infection with coronavirus because of the inflammation, scarring and lung damage those conditions cause, Johns Hopkins Medicine reported.
        Covid-19 affects a person's airway and lungs, but those organs work together to provide the body with oxygen. When the lungs are overburdened with an infection, the heart has to work harder, which exacerbates the challenges of people already living with heart disease.

        The immunocompromised

        According to the CDC, many conditions can cause a person to be immunocompromised, including cancer treatment, smoking, bone marrow or organ transplantation and immune deficiencies. Poorly controlled HIV or AIDS and prolonged use of man-made steroid hormones or other immune-weakening medications can also hamper a person's immune function.