Influenza—usually called the flu—is a contagious respiratory illness caused by influenza viruses. It circulates every year, most often during the colder months in Canada. For many people, the infection causes several difficult days followed by gradual improvement. For others, especially those at higher risk, influenza can lead to pneumonia, worsening of a chronic condition, hospitalization or death.
Knowing what flu can look like helps you decide when home care is reasonable, when to call a healthcare professional and when symptoms require emergency attention. It is equally important to recognize that symptoms alone cannot always distinguish influenza from COVID-19 or another respiratory infection.
This guide provides a practical overview. It does not diagnose an individual illness, and anyone who is worried about their symptoms should seek personalized medical advice.
What does influenza usually feel like?
Flu symptoms often begin suddenly. Someone may feel reasonably well in the morning and develop significant fatigue, fever and body aches later that day. Common symptoms include:
- fever;
- cough;
- muscle or body aches;
- headache;
- chills;
- unusual tiredness or weakness;
- sore throat;
- runny or stuffy nose; and
- reduced appetite.
Children may also have nausea, vomiting or diarrhea. These digestive symptoms can occur in adults but are more common in children. Not everyone with influenza develops a fever, particularly older adults or people with weakened immune systems.
Symptoms usually appear one to four days after exposure. Most people recover within seven to ten days, although a cough or fatigue can continue longer. A person who seems to improve and then becomes significantly worse should be reassessed because this pattern can signal a complication or a second infection.
Flu, a cold or COVID-19?
Influenza and the common cold can share cough, sore throat and nasal congestion. A cold is often milder and tends to develop gradually. Influenza is more likely to begin abruptly and cause fever, pronounced aches, headache and exhaustion. These are tendencies, not a reliable home test.
COVID-19 can look very similar to influenza. Both can cause fever, cough, fatigue, aches and sore throat, and either illness can range from mild to severe. Other respiratory viruses can create the same uncertainty. A person can also be infected with more than one virus at the same time.
Testing may be useful when the result would change treatment, isolation decisions or protection of a vulnerable person. Available testing and public-health advice can change. If you are unsure whether you should test or how long to avoid close contact, follow current Ontario guidance or contact a healthcare professional.
How influenza spreads
Influenza spreads mainly through respiratory particles released when an infected person breathes, talks, coughs or sneezes. Transmission is more likely during close indoor contact, especially where ventilation is limited. The virus can also reach the eyes, nose or mouth after contaminated hands touch the face, though shared air and close contact are major concerns.
A person can begin spreading influenza about one day before symptoms appear and may remain contagious for approximately five days afterward. They are generally most contagious during the first three days of illness. Children and people with weakened immune systems may spread the virus for longer.
This timing explains why influenza can move quickly through a household. Someone may expose others before realizing that the first headache or tired feeling is the beginning of an infection.
Who is more likely to become seriously ill?
Anyone can develop complications, but some people require closer attention. Higher-risk groups include:
- babies and young children;
- adults aged 65 and older;
- pregnant people and those who recently gave birth;
- residents of long-term care or other congregate facilities;
- people with asthma, chronic lung disease or heart disease;
- people with diabetes, kidney disease or certain neurologic conditions;
- people living with cancer, HIV or another cause of reduced immunity;
- people taking immune-suppressing medicines; and
- people with other significant chronic health conditions.
People in these groups should contact a healthcare professional early when flu symptoms begin, even if the illness initially seems manageable. A clinician may recommend closer monitoring, testing or prescription antiviral treatment based on the person’s risk, symptom severity and timing.
How is flu diagnosed?
A clinician can often make a working diagnosis from symptoms, the physical examination and knowledge of what infections are circulating. They may listen to the lungs, measure oxygen levels and assess hydration. Not every patient needs a laboratory test.
A nasal or throat sample may be helpful when confirmation would influence treatment or infection-control decisions. Testing is more likely to matter for people who are seriously ill, are at higher risk, live in a setting with an outbreak or need hospital care. A negative rapid test does not always rule out influenza, so clinicians interpret results alongside the overall picture.
Chest X-rays and blood tests are not routine for uncomplicated flu. They may be ordered when pneumonia, dehydration or another complication is suspected. Shortness of breath, low oxygen, chest pain, a worsening cough or symptoms that return after initial improvement may prompt additional evaluation.
Caring for mild flu at home
Most otherwise healthy people with mild influenza can recover at home. The main goals are rest, hydration, symptom relief and preventing spread.
Rest and reduce activity
Your body needs energy to respond to infection. Sleep, postpone strenuous exercise and do not pressure yourself to resume a full schedule immediately. Returning to activity gradually is sensible, particularly if fever or significant fatigue has been present.
Stay home while acutely ill. This protects co-workers, classmates, customers and other people you may encounter. If you must be around others, follow current public-health advice and consider added precautions such as a well-fitting mask and better ventilation.
Drink enough fluids
Fever, rapid breathing, vomiting or diarrhea can contribute to dehydration. Water, soup, oral rehydration drinks and other tolerated fluids can help. Take small, frequent sips if nausea makes a full glass difficult.
Signs of dehydration include a very dry mouth, dizziness, reduced urination, dark urine or unusual sleepiness. Infants may have fewer wet diapers or no tears when crying. Significant dehydration requires medical assessment.
Ease fever and aches carefully
Acetaminophen or ibuprofen may help some people with fever, headache and muscle pain. These medicines are not right for everyone. Kidney disease, liver disease, stomach ulcers, blood thinners, pregnancy, dehydration and other factors can change what is safe. Follow the label and ask a clinician if you are uncertain.
Check the ingredients of multi-symptom cold-and-flu products. Several products may contain acetaminophen, and combining them can lead to an accidental overdose. Do not give aspirin to a child or teenager with a viral illness unless a clinician has specifically directed it because of the risk of Reye syndrome.
Manage cough and sore throat
Warm fluids, honey for people older than one year, throat lozenges and humidified air may provide comfort. Never give honey to a baby under 12 months because of the risk of infant botulism.
Over-the-counter cough and cold products have age restrictions and may interact with prescriptions or health conditions. They should not automatically be given to young children. Ask a clinician for age-appropriate advice, and avoid assuming that “natural” remedies are risk-free.
What are antiviral medicines?
Prescription antivirals act against influenza viruses. They are different from antibiotics, which target bacteria. Antivirals may shorten illness or reduce complications for selected patients, particularly when treatment begins early.
A clinician may consider antiviral treatment for someone who is hospitalized, has severe or progressive symptoms, belongs to a higher-risk group or has another reason that makes complications more likely. Treatment decisions should not be based only on a home diagnosis. The medicine, dose and length of treatment depend on factors such as age, kidney function, pregnancy, symptom timing and the viruses circulating.
If you are at higher risk, contact a healthcare professional promptly instead of waiting several days to see whether symptoms become severe. Early contact gives the clinician more options. People with serious or worsening illness may still benefit from assessment and treatment even if symptoms did not start recently.
Antivirals can cause side effects and are not a substitute for annual vaccination. They are one treatment tool, not a reason to skip prevention.
Why antibiotics do not treat influenza
Influenza is viral, so antibiotics do not cure it, shorten it or prevent it from spreading. Using antibiotics when they are not needed can cause diarrhea, allergic reactions and other side effects. It also contributes to antibiotic resistance, making bacterial infections harder to treat in the future.
An influenza infection can sometimes be followed by a bacterial complication such as pneumonia, a sinus infection or an ear infection. In that situation, a clinician may prescribe antibiotics for the bacterial illness—not for the influenza virus itself. Symptoms that worsen after improvement, a returning fever, increasing shortness of breath or new focal pain should be assessed.
Never use leftover antibiotics or someone else’s prescription. The drug may be wrong for the condition, the dose may be inappropriate and partial treatment can complicate future care.
When should you call a clinic?
Contact a healthcare professional for advice if:
- you are in a higher-risk group and develop flu-like symptoms;
- symptoms are getting worse rather than gradually improving;
- fever lasts longer than expected or returns after resolving;
- you cannot drink enough or notice signs of dehydration;
- a cough is severe, persistent or associated with wheezing;
- a chronic condition such as asthma, diabetes or heart disease is becoming harder to control;
- you are unsure which over-the-counter medicine is safe;
- an infant, older adult or immunocompromised person is ill; or
- you need guidance about testing, antiviral treatment or returning to work or school.
Telephone advice can help determine whether an in-person assessment is needed. When calling, be ready to explain when symptoms began, current temperature, breathing symptoms, medical conditions, medications and whether there has been contact with a vulnerable person.
When flu symptoms may be an emergency
Call 911 or seek emergency care immediately for severe symptoms. Warning signs in adults can include:
- trouble breathing, severe shortness of breath or blue-grey lips;
- persistent pain or pressure in the chest;
- new confusion, inability to wake or a marked change in alertness;
- fainting, severe weakness or inability to stand;
- signs of severe dehydration, such as almost no urination;
- a seizure;
- coughing up blood; or
- rapid deterioration or any symptom that feels life-threatening.
For babies and children, emergency warning signs can include:
- struggling to breathe, ribs pulling in with breaths or grunting;
- blue or grey colouring around the lips or face;
- being difficult to wake, unusually limp or not interacting;
- a seizure;
- severe dehydration, no tears or very few wet diapers;
- inability to drink or repeated vomiting;
- extreme irritability that cannot be settled; or
- symptoms that improve and then return with greater severity.
This is not an exhaustive list. Trust your judgment when someone appears seriously unwell. Do not delay emergency care while trying to decide whether the illness is definitely influenza.
Protecting the rest of the household
If possible, the sick person should rest in a separate, well-ventilated room and reduce close face-to-face contact. Avoid sharing drinking glasses, utensils and towels. Wash hands often, especially after handling tissues, and clean frequently touched surfaces.
Improve indoor air by opening windows when practical or using appropriate filtration. A well-fitting mask can add protection during close contact, particularly when someone in the home is medically vulnerable. Caregivers should monitor themselves for symptoms.
Do not send a child to school or daycare while they are acutely ill. Adults should also avoid returning to work simply because fever medicine temporarily lowers their temperature. Follow the policies of the school, workplace or care setting and any current public-health guidance.
Preventing influenza
Annual influenza vaccination is recommended for most people aged six months and older. The vaccine is updated because circulating strains change and immunity decreases over time. It cannot prevent every infection, but it can lower the likelihood of illness and serious outcomes.
Other useful habits include:
- washing or sanitizing hands regularly;
- avoiding touching the eyes, nose and mouth with unclean hands;
- covering coughs and sneezes;
- improving ventilation in shared indoor spaces;
- cleaning high-touch surfaces when someone is sick; and
- staying home when ill.
People who live with a newborn, an older adult or someone with reduced immunity can help protect that person by staying current with vaccination and taking symptoms seriously.
Returning to normal activity
Recovery is not always a straight line. Fever and aches may settle before energy and concentration fully return. Start with light activity and increase gradually. Stop and seek advice if exertion produces chest pain, unusual breathlessness, dizziness or a racing heart.
A lingering cough can occur after a viral infection, but it should trend toward improvement. A cough that is worsening, accompanied by a new fever, produces blood or interferes with breathing needs assessment. People in physically demanding jobs may need more recovery time than those able to work quietly from home.
The bottom line
Flu often begins suddenly with fever, cough, body aches, headache and marked fatigue, but symptoms overlap with COVID-19 and other respiratory illnesses. Most mild cases improve with rest, fluids and careful symptom management. People at higher risk should seek advice early, and severe breathing difficulty, chest pain, confusion, dehydration or rapid deterioration requires emergency care.
Town Care Walk-In Clinic can assess non-emergency respiratory symptoms and advise on testing or treatment when appropriate. Call (416) 792-2654 with general questions or review current walk-in hours before travelling. If symptoms are severe or life-threatening, call 911.
For current federal information, visit the Government of Canada’s guide to flu symptoms, spread, prevention and treatment.
Medical information is for general education and does not replace professional advice. Recommendations can change, and individual care depends on age, medical history, medications and symptom severity.