Influenza Virus



Influenza Virus 101 


Although the first influenza virus was not isolated until the 1930s, there is evidence that influenza coexisted with humans long before we knew what it was. Historians have documentation of a likely influenza outbreak as early as 1510, but researchers believe it has been hanging around with humans since the 9th century. Perhaps the most infamous influenza outbreak was the 1918 pandemic, sometimes referred to as the Spanish flu. However, "Spanish flu" is a misnomer resulting from Spain's frequent press coverage of the deadly illness, which gave the erroneous impression Spain suffered more from the pandemic than other countries.

Now influenza is worldwide, with outbreaks in the United States occurring every winter. Influenza's presence during colder months is not because it prefers snow sports over sunbathing but because of human proximity. It is easier for the virus to get from person to person when everyone is indoors trying to stay warm. The closer you are to someone who hurls the virus at you through a sneeze, the more likely you will become infected.  

Once infected, most people mistake influenza for a bad cold, but some will become quite ill beyond a runny nose, headache, and fever. For many, the influenza virus is nothing more than a minor nuisance, but this single-stranded RNA virus causes hundreds of thousands of hospitalizations each year and thousands of deaths every flu season. If you don't care about the flu, you ought to. As common as it is, you will likely become ill with influenza or care for someone with it sooner or later.  


little girl not feeling well with flu like symptoms flu shot family with influenza vaccine Caregiver caring for an elderly patient with influenza

Influenza Symptoms

The influenza virus can be mild to severe, with the really young and really old being most at risk of severe disease and associated complications. Not everyone who becomes infected with the virus will have the same symptoms. Influenza causes upper respiratory infections, usually involving the nose and throat, but sometimes infects the lungs.  

Symptoms of influenza are:  

• Fever 

• Cough 

• Sore throat 

• Runny nose and nasal congestion 

• Muscles and body aches 

• Fatigue 

• Headache 


It is common to have influenza without a fever, and many individuals will only have a couple of the listed symptoms. However, some unfortunate people will exhibit all of the symptoms. In some cases, you may experience vomiting or diarrhea, but these uncommon manifestations tend to occur only in children. If your child experiences diarrhea and vomiting due to influenza infection, pay attention to signs of dehydration. Dehydration is already a risk of influenza due to possible fluid intake reduction and fevers contributing to water loss. 

On average, initial symptoms present two days after exposure but range from one to four days. Flu symptoms last around five to seven days but can be shorter if you have had the flu shot or longer if you have complications. You are at risk of complications if you have certain underlying conditions like asthma, diabetes, kidney disease, cancer, or HIV/AIDS. Due to well-known health disparities, like lower vaccination rates, racial and ethnic minorities are at a greater risk of hospitalizations, ICU admissions, and death associated with influenza.

Causes of Influenza

There are four types of influenza: influenzas A, B, C, and D. Influenza A and B are the types responsible for seasonal epidemics or flu season. Influenza C infections are limited to minor upper respiratory infections in children, while D prefers to infect cattle and not humans. Influenza A and B circulate during flu season and infect five to twenty percent of the United States population during these annual epidemics.viii 


Influenza A 

Influenza A is the predominant type and causes most cases of flu. A wide variety of birds can become infected with it, ensuring worldwide prevalence. Because of the large reservoir in wild birds, influenza A also has ample opportunity for different subtypes to coinfect a single bird and reassort into new subtypes.  

Influenza A has two proteins on its surface, hemagglutinin (H) and neuraminidase (N). There are 18 H subtypes (H1 through H18) and 11 different N subtypes (N1 through N11). Each H and N subtype can reassort into new H and N combinations. Reassortment occurs when two different influenza viruses simultaneously infect a single host and share their genetic data. So far, 130 distinct reassortments have been identified, with most occurring in wild birds.ix  

Currently circulating human subtypes are H1N1 and H3N2. Subtypes can be broken down further into groups and subgroups (also called clades and subclades) based on their gene sequences. Small changes in these sequences occur over time as the virus replicates until your immune system no longer recognizes it. At this point, the virus has effectively evaded immunity, which is bad for humans but good for the virus from an evolutionary standpoint.   

Although influenza A and B both cause epidemics, influenza A is the only known type of flu virus that causes pandemics. An epidemic happens when an infectious disease spreads throughout a community during a particular time. A pandemic is an epidemic that spreads beyond a specific region and into other countries or continents. Pandemics happen when a new virus or bacteria emerges that can spread efficiently and to which humans have little or no immunity.  


Some well-known and well-documented influenza pandemics caused by type A influenza are:x  

• H1N1: 1918, causing 50 million deaths globally and 675,000 in the US  

• H2N2: 1957, causing 1.1 million deaths globally and 116,000 in the US 

• H3N2: 1968, causing 1 million deaths globally and 100,000 in the US 

• H1N1: 2009, causing 151,700 to 575,400 deaths globally and 12,469 deaths in the USxi 


Influenza B 

Influenza B is not broken down into subtypes but into lineages and then further divided into groups and subgroups (or clades and subclades). There are two influenza B lineages: Victoria and Yamagata. Type B evolves slower than type A and does not cause pandemics but is still responsible for seasonal epidemics. Influenza B infects just humans and seals,xii which is a much smaller reservoir than the wide variety of birds that carry influenza A. The inability of type B to cause pandemics may be partly due to the limited reservoirs available to the influenza B virus, therefore creating fewer opportunities for mutating into new or novel strains.  


How Influenza Spreads 


Influenza is a highly infectious virus with a reproductive number, R0 (pronounced R naught), between one and two.xiii This means that for every one person infected, they infect one or two others. R0 is the average number of cases caused per one infected individual and is a way to estimate the contagiousness of an infectious disease.  

The flu spreads when droplets from an infected person exit through the mouth or nose and find their way to a healthy individual. If the viral-laden droplets make it into an uninfected person's nose, mouth, or eyes, they may become sick with influenza. People carrying the flu virus can infect others up to six feet away.xiv 

Less commonly, flu can spread through fomites (surfaces) such as doorknobs or utensils. If a child with the flu sneezes into their hand and then touches a toy, they may deposit viral particles onto the surface. If a healthy parent later picks up that toy and then touches their nose, mouth, or eyes, they can potentially innoculate themselves with the flu virus and cause an infection.  

Once infected with influenza, you are most contagious during the first few days after symptoms commence but can infect others one day before becoming sick and up to seven days after. Asymptomatic infections, or infections that do not present with symptoms, occur in about half of influenza cases and can still spread the disease.xv Asymptomatic and pre-symptomatic transmission should be considered when evaluating influenza prevention strategies.  

Diagnosing Influenza

If you have influenza symptoms like a headache, fever, and cough during influenza season, there is a good chance you have contracted influenza. However, flu can present outside of influenza season, and other respiratory viruses can mimic the flu. Your doctor cannot say for certain if you have influenza based on symptoms alone. A definitive flu diagnosis requires diagnostic testing.  


In most cases, resting at home is sufficient to recover from flu-like illnesses, and clinical tests are unnecessary. If your doctor needs to know if you are infected with the influenza virus, there are several tests they can perform. Some flu tests offer quick results but may miss positive samples, while others may take longer to receive a result but are unlikely to give a false negative.  


Rapid influenza diagnostic tests are the most widely used method to detect influenza infections, and just like the name suggests, they provide prompt results, usually within fifteen minutes. These relatively low-cost tests are quick and easy to use, making them simple enough for a nurse to perform right in the clinician's office. The drawback is that rapid tests are generally less sensitive than ones sent to a lab and conducted by trained laboratory scientists.  


Less sensitive tests are more likely to miss true positive results, which means a sick patient may be told they are negative for influenza when they in fact do have the disease. Doctors are aware of the sensitivity and consider it when diagnosing and treating their patients. Your doctor may choose to initiate treatment, especially if you are high-risk, even if a rapid test yields a negative result. Your doctor can also send your sample to the lab for more sensitive testing.  

RT-PCR (reverse transcription-polymerase chain reaction) tests are sensitive enough to detect influenza in most positive samples and specific enough to determine negative results in most samples where the influenza virus is absent.xvi These molecular tests tend to be more expensive and take longer due to the necessity of sending them out to a lab with PCR testing capabilities.  

Treating Influenza Infections

For healthy individuals who do not fall into a high-risk category treating symptoms at home is usually adequate. Continue to drink plenty of fluids and eat enough to satiate hunger. Although over-the-counter medications and other symptom-reducing methods won't shorten the duration of your illness, they may make you more comfortable. Ibuprofen and acetaminophen can help with headaches, fevers, and sore throats. Talk to a doctor before administering any medication to children or if you are taking something for the first time. Adults and children can continue normal activity levels if they feel well enough but should avoid people to prevent spreading the virus.  

There is no cure for the flu virus, and it cannot be treated with antibiotics. However, antivirals may be indicated for some patients with underlying conditions. Antivirals are most effective when initiated within 48 hours of symptom onset. You should contact a doctor if you have flu-like symptoms and are immunocompromised or considered high-risk. It is also important to speak with a doctor if your symptoms are not improving or are unusually severe. Seek immediate medical help if you are lethargic, losing consciousness, or having trouble breathing. Infants under three months should see a doctor straight away if they have a fever of 100.4 or higher.  

Preventing Influenza Transmission

Obtaining an annual flu shot is the best way to protect yourself and others from contracting influenza. In addition to your flu shot, practicing appropriate health and hygiene habits can decrease your chances of becoming sick this flu season. The flu virus changes every year, rendering your immunity from the last flu season, whether induced by a vaccine or naturally acquired from infection, unable to fight off this year's strain.  


Although most individuals with influenza recover just fine, certain demographics are at risk of serious complications or death. Protecting yourself from contracting the flu means you are also protecting those at serious risk of influenza complications. The most effective and simplest way to protect yourself and others is to get your flu shot every year.  


The Annual Flu Shot 


The best prevention for influenza is the annual flu shot, which protects against both A and B types. Scientists and public health officials study influenza patterns and strains to predict which ones will cause the next seasonal flu outbreak. Because the flu virus changes frequently, a new flu shot is developed each year to ensure efficacy.  

Getting your flu shot before flu season is in full swing gives you the most out of your flu shot. Vaccination before the end of October gives your immune system plenty of time to build up antibodies against the flu before the virus spreads throughout the community. However, flu activity can last into May, so a flu shot later in the season will still benefit you.  

Everyone over six months should get an annual flu shot, with rare exceptions. Talk to a doctor if you are unsure if the flu shot is right for you or your child. If you live with or care for someone in which the flu shot is contraindicated, prioritize your flu shot to diminish your chances of bringing the virus home to your loved one.  

People over 65 years of age may benefit from a type of flu shot with increased potency. Older individuals do not always elicit as robust of an immune response as their younger counterparts. Scientists have designed flu shots specific for the geriatric population to ensure a safe and effective vaccine. Talk to a doctor to determine if the higher potency flu shot is right for you.  





Healthy Habits to Avoid Getting the Flu 


The flu virus can transmit when you touch contaminated surfaces and then touch your face. Wash your hands often with soap and water during flu season. If soap is unavailable, use an alcohol-based sanitizer. Refrain from touching your eyes, nose, or mouth with unwashed hands, and avoid close contact with anyone who has flu-like symptoms. Frequently clean high-touch surfaces such as doorknobs, children's toys, utensils, or countertops to reduce fomite transmission.  

More commonly, flu spreads through droplets discharged from infected individuals into the air. If you have flu symptoms, stay home. If you must leave your house, wear a mask and always cough or sneeze into your elbow or a tissue. Although the probability of getting the flu, spreading it, or becoming severely ill from it significantly decreases after a flu shot, you can still catch and share it with others.  

Don't underestimate maintaining your overall health. Engaging in healthy habits like staying active, prioritizing adequate sleep, and eating right may improve your ability to fight off cold and flu viruses. If you are a smoker, consider quitting. Consume alcohol in moderation and avoid second-hand smoke.  

Our board certified doctors can treat a wide variety of medical conditions through SkyMD®. Common conditions that can be diagnosed and treated through our online platform include but are not limited to:

Our Providers

Dr. Yarledis Salcedo
Dr. Yarledis Salcedo
of experience
since April, 2022 with SkyMD
Dr. Lisa Hiller
Dr. Lisa Hiller
22 of experience
since February, 2022 with SkyMD
Dr. Cshantara Audap
Dr. Cshantara Audap
6 of experience
since May, 2022 with SkyMD

Ready to submit your first visit on SkyMD®?

Accreditations and Certifications