The influenza virus is a sphere-shaped virus with a diameter one ten-thousandth of a millimeter (100nm, 1/10um), encased in a lipid membrane called an envelope. The virus is also sometimes observed in a thread shape, 1,000nm long. A virus can multiply itself, becoming 100 viruses in eight hours. Over a day, these viruses can number one million, if they are able to infect cells and multiply.
Flu viruses that infect humans are categorized into three types: A, B and C. Type A and B can infect humans and spread rapidly. Type C only causes mild cold-like symptoms. There are type A influenza viruses that can infect animals such as birds, pigs and horses as well as humans. Type B can only infect humans.
Influenza viruses are also classified into a subtype by the antigens on the surface of the virus. For example, H1N1 or H3N2 are the subtypes of type A influenza. H1N1 has one hemagglutinin and one neuraminidase protein on the surface of the virus. These proteins, Hemagglutinin (HA) and Neuraminidase (NA), are important for a virus's infection capabilities.
The big difference between influenza and a cold is the severity of symptoms and the infectiousness of the virus. Influenza is a disease, not surprisingly, caused by the influenza virus. While there are multiple types of the virus, types A and B are very infectious and can trigger widespread infections. Symptoms typical of influenza, include a fever over 38 degrees, headache, muscle pain, and physical fatigue. Symptoms develop suddenly, after a short latent period of one to three days from infection. Upper respiratory symptoms such as a cough and runny nose continue until recovery about a week later.
An infected person can spread the virus 1 day before symptoms appear, and infectiousness peaks within 24 to 48 hours then drops sharply, but this can last from five to ten days. Infants may spread the virus for a longer time.
On the other hand, a common cold is not one particular virus, but approximately 10 different viruses with a total of 200-300 types. Symptoms are not serious in general such as a runny nose although symptoms differ depending on the virus. The infectiousness of the cold virus is usually not as strong as influenza.
In one study, those infected and those who were not were put together in one room to examine the infection route of rhinovirus. While the participants exposed each other to mucous and saliva, even kissing, the infection did not spread widely. Symptoms are generally mild.
|
Comparison Point |
Influenza |
Cold |
|---|---|---|
|
Initial symptoms |
Chills, headache, sudden fever |
Dry nasopharynx, sneezing |
|
Major symptoms |
Chills, fever, physical fatigue, headache, backache, muscle pain, arthralgia, nasal congestion, cough, phlegm, sore throat |
Sneezing, Runny nose, Nasal congestion, Cough, Sore throat, Slight fever, Physical fatigue |
|
Chills |
Severe |
Minor, very short period |
|
Fever (period) |
38~40 degrees (3~4 days) |
None or slight fever |
|
Fatigue |
Severe |
Almost none, minor |
|
General symptoms |
Severe |
None |
|
Complications |
Bronchitis, ear infection, sinusitis, croup, influenzal pneumonia, bacterial pneumonia, liver damage, febrile convulsion, reye syndrome, guillain-barre syndrome, encephalitis/ encephalopathy (especially infants), myocarditis, renal failure |
Rare: (sinusitis, bronchitis, pneumonia, ear infection, conjunctivitis, meningitis) |
|
Season |
Influenza virus (around November-April) |
Rhinovirus (winter), Adenovirus (all year), Coronavirus (winter-spring), RS virus (around November-March), Parainfluenza virus (around March-July), Bacteria, Mycoplasma |
|
Infectiousness |
Strong, increase rapidly |
Weak, viruses gradually increase |
|
Immediate diagnosis |
Necessary |
Not necessary |
Influenza often enters homes from children infected at school. Morbidity of influenza peaks in early childhood since the development of the immune system is still in progress. Morbidity lowers as children get older through development of the immune system and vaccinations.
Influenza generates a vast number of patients in a short time, involving all age during influenza season for the following reasons:
Mortality of the elderly is higher than other ages although their morbidity is low. This is because resistance lowers with age, and the risk factors for influenza is high due to other diseases.
High risk group - People who likely to experience severe influenza infections or fatal complications
How does the influenza virus travel and infect people? Influenza is thought to be spread over three different routes: droplets, airborne, and contact transmissions. Among them, droplet transmission is said to be the main route. Let's take a look at how each transmission is made.
When we sneeze or cough, small particles of mucous containing influenza viruses can be spread into the air. These particles are called droplets and the number of the particles can reach approximately two million per sneeze and 100 thousand per cough. The particles directly enter our lungs to cause infection. People 1-1.5 meters from an infected person are very susceptible to breathing in droplets.
Viruses are transmitted through hands by touching environmental surfaces or objects contaminated by droplet spray, and other objects. Viruses enter through mucous membranes from the hands when we unconsciously touch our eyes, nose or mouth. This is called contact infection. Since viruses can survive for a long time in a dry environment, infection can spread by people touching phones, doorknobs, tableware and or hand rails which have been used by those with the flu.
When droplets carrying a virus evaporate in the air, the virus can be suspended in the air for long periods of time. People can get infected by inhaling these floating nuclei. This is called airborne infection or droplet nuclei infection. Viruses are infectious for a longer period in dry environments with lower temperatures, and particles tend to float relatively longer in these conditions as well. Airborne infection occurs under these conditions. It is important to manage air conditioning, ventilation, temperature and humidity to prevent airborne infection.
|
|
Droplet Spray |
Droplet Nuclei |
|---|---|---|
|
Infection pattern |
Droplet infection |
Airborne infection |
|
Particle diameter |
5μm or bigger |
5μm or smaller |
|
Falling rate |
30~80cm/sec |
0.06m~1.5cm/sec |
|
Moist particle |
Yes |
No |
|
Range |
Short (Approx. 1m) |
Long |
Reference: CDC Guideline for Isolation Precautions 1996
