There are usually a few indicators when you’re sick. It’s common to feel noticeable symptoms with any illness, such as a fever, headache, shortness of breath, stomach or chest pain, and coughing or sneezing.

But sometimes, you can be sick without even knowing it – whether you have a viral infection like the flu or a chronic condition like high blood pressure.

These types of illnesses are called asymptomatic, meaning they present without symptoms. Here’s what you need to know about asymptomatic illnesses, how they differ from presymptomatic illnesses, and why these terms are important to understand.

Examples of asymptomatic illnesses

There are many long-term medical conditions and short-term illnesses that have no symptoms, says epidemiologist Aaron E. Glatt, MD, chair of medicine at Mount Sinai South Nassau.

In fact, millions of people around the world have what are called silent, or latent, infections, Glatt says. They are called this because some bacteria and viruses transmit infections for which there may be no symptoms at all.

"People can go through their whole lives having an asymptomatic illness such as tuberculosis, which we call latent tuberculosis," Glatt says.

Moreover, a study published in The Lancet in 2014 found that 77% of flu infections didn't have any symptoms. The lack of symptoms may be due to having preexisting immunity to the infection.

Some chronic illnesses also have no symptoms, such as high blood pressure (hypertension), which affects more than 30% of adults in the US. In fact, it is often called the "silent killer" because it can develop over time and damage your body without you even knowing it.

The following common diseases can occur without any symptoms:

  • Hypertension (high blood pressure)
  • Hepatitis B and C
  • Herpes simplex virus (HSV)
  • Type II diabetes
  • Glaucoma
  • Osteoporosis
  • Respiratory diseases, like the flu or COVID-19

Asymptomatic illnesses can still be contagious

Most asymptomatic illnesses, like hypertension or diabetes, are not contagious.

However, if you do have a contagious illness, like the flu or common cold, and never have any symptoms, you can still spread it to other people.

For example, one in three people with the flu are asymptomatic, according to a study published in Public Health Reports in 2009. Accordingly, some researchers believe that the rate of influenza spread by asymptomatic people is about one third to one half of the rate spread by people with symptoms.

However, the evidence for asymptomatic influenza transmission is scant, as it's difficult to track who has the flu if they don't have any symptoms.

"With many illnesses, especially viral illnesses, you're generally contagious in the day or two beforehand and during the first few days of illness," Glatt says. "We don't really know how many people without symptoms spread the illness, but it's certainly possible."

Asymptomatic vs. presymptomatic

It's important to understand the difference between these two medical terms. Asymptomatic means there are no symptoms through the duration of your illness. Presymptomatic means that there is an initial period without symptoms, but symptoms will develop later on.

For example, both the flu and cold are most contagious within the first three or four days of catching each respective virus, and this is often before symptoms occur, or when people are presymptomatic.

The coronavirus outbreak has also led to a greater understanding of these terms. It's estimated that about 40% of COVID-19 transmissions occur when people are presymptomatic, according to the US Centers for Disease Control and Prevention.

Moreover, up to 45% of COVID-19 infections may be asymptomatic, as symptoms may simply never develop for some people. The actual rate of asymptomatic transmissions of COVID-19 is not yet known, according to the World Health Organization (WHO).

This is part of the reason why screening tests are so important. If you're presymptomatic or asymptomatic, you won't know you have an illness. Without getting a medical test, you would go about your life as normal, possibly further spreading contagious illness.

In many cases, these tests can be lifesaving - both for COVID-19 and other illnesses. For example, some types of cancer show no indication of symptoms until it spreads in the body, and by that point, it may be more difficult to effectively treat.

That's why to help detect breast cancer, the American Cancer Society recommends annual mammograms for women who are 45 to 54 years old, and every other year for women 55 and older. Similarly, the best way to know if you're at risk of heart disease is by having your doctor check your blood pressure readings at least once a year.

Overall, routine medical check-ups and screening tests are the most effective measure for preventing health complications from asymptomatic or presymptomatic illnesses.

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