Influenza (Flu)

What is the Flu (influenza)?

The flu is an illness caused by a virus (germs) that usually affects the nose, throat, and lungs. It is easily spread from person to person. It can cause mild to severe illness, and at times can lead to death. The typical flu season starts in October and ends in late Spring, with illnesses peaking in December-February. There are two types of viruses, flu A and flu B, that usually circulate within a season. The best way to prevent flu is by getting a vaccine each year.

Anyone can get the flu, but there are some groups of people who have a higher risk of more severe symptoms and serious complications. Those at higher risk include:

  • Adults 65 years and older
  • young children (less than 5 years old but especially those younger than 2)
  • Pregnant women
  • Those with other serious health conditions including, but not limited to:
    • Asthma
    • Diabetes
    • Heart Disease
    • Weakened immune system (due to diseases like HIV/AIDS or cancer, or some medications)
    • Populations at higher risk for flu


Three simple steps can help protect you and those around you:

  1. Get Vaccinated! The best way to prevent the flu is to get your flu shot every year. Everyone over the age of 6 months should get vaccinated each year. While it is best to get your annual flu shot early in the fall, receiving a flu vaccine at any time during the flu season is beneficial. Getting a flu shot not only helps protect you from getting ill but may also protect those around who are at higher risk of severe illness. Flu vaccine can usually be given concurrently with other vaccines. Visit the Colorado Dept. of Public Health & Environment (CDPHE) website to find vaccination locations near you.

  2. Practice everyday preventive actions! It is always recommended to avoid close contact with people who are sick. If you are sick, stay home except to seek medical attention. Wash your hands with soap and water. If soap and water are not available, use alcohol-based hand sanitizer. Cover your coughs and sneezes with a tissue or your elbow. Clean and disinfect high-touch surfaces frequently. 

  3.  Take antiviral medications as prescribed by your doctor! Antiviral medications can be prescribed by your doctor to help treat your symptoms. They will not cure the flu but can make symptoms milder and shorten the time you are sick. 


The flu can cause mild to severe illness. These are the most common symptoms of the flu.

You may experience some or all symptoms such as:

  • Fever or feeling of feverish/chills (it is important to note that not everyone with the flu will have a fever)
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Body aches
  • Headaches
  • Fatigue (tiredness)
  • Vomiting and diarrhea (more common in children than adults)

Learn more about the difference between cold and flu. 

Flu Surveillance

Annual flu surveillance is a critical component of public health infrastructure. Public health uses a variety of surveillance activities to identify which flu strain types are circulating in the community, monitor for novel strains, and inform planning for the next flu season. We also use this data to understand characteristics of the current circulating strains, such as virulence or severity, transmissibility, most highly affected populations, etc.

How is flu surveillance conducted in Colorado?

  • Hospitalizations due to flu and flu-associated pediatric deaths can be used to measure the severity of the circulating flu strains
  • Tracking flu outbreaks in long-term care facilities and correctional facilities, monitoring influenza-like illness (ILI) patient visits to outpatient clinics and emergency departments, and collecting sample positivity data from sentinel laboratories can all be used to estimate the incidence rate of flu circulating within the community
  • Sentinel testing of positive flu samples can tell us the predominant type of flu circulating and help us monitor for new or novel strain types

Flu data is collected and managed at the state level through the Colorado Department of Public Health and Environment. For current and previous Colorado flu surveillance data, visit the CDPHE Flu Surveillance page at

To report suspected or confirmed flu outbreaks at your facility, please contact the DDPHE Disease Intervention and Epidemiology program at 720-865-5767 or

Find A Flu Vaccine

Visit the Colorado Dept. of Public Health & Environment (CDPHE) website to find vaccination locations near you.