Flu (Influenza): Epidemic Flu, Endemic Flu, and Pandemic Flu Viral Infection?

The study published 2019 suggests annually average 389,000 (range 294 000-518 000) individuals die from a respiratory complication caused by viral infection every year. The mortality is more common among people older than 65 years.¹

Influenza is also known as the flu. Influenza is a seasonal viral infection caused by influenza viruses. The seasonal flu infection is widely spread in the local population because of the rapid transmission of viruses from one individual to another individual. Flu symptoms are running nose, throat pain, cough, and body ache. Occasionally a few patients also suffer from fever. Flu is a seasonal disease.

The fall and winter in the USA are considered flu season. Most cases are seen between December and February, though the flu virus infection in lesser number observed throughout the year.

Yes, the Influenza virus is a single-stranded RNA virus. It is spherical (about 100 nm in diameter), or cylindrical.² Coronavirus is different than the influenza virus. Coronavirus is also an RNA virus and has a club like spikes on its surface.³ Influenza and coronavirus cause upper respiratory infection in chickens, cows, pigs, and humans. Coronavirus mutation has caused a severe epidemic, endemic, and pandemic infection resulting in SARS, MERS, and COVID-19 infection.

The flu virus is known as the influenza virus. There are three types of influenza virus known as Influenza virus A, B, and C.⁴ Most seasonal flu is caused by type A and  B influenza virus. Type A influenza virus is a sub type into H and N influenza virus depending on a surface antigen that is attached to the virus envelope. The surface antigens are identified as H and N protein molecules. 

Endemic flu viral infection is observed in the local population that is spread within the city or district boundary. Simultaneously, there could be multiple geographical pockets of such diseases that are spread around the state or country. Endemic form of viral infection is seasonal and repeats at an expected frequency in a particular geographical area.⁵

The seasonal viral infection, such as flu, becomes epidemic flu when the infection spreads beyond the city and district throughout the state or within the country. The endemic flu viral infection becomes epidemic flu when the viral infection spreads from human to human of all ages from the endemic area to the entire district or state.⁵

The most seasonal viral infection is endemic or epidemic. Occasionally epidemic infection rapidly spreads from one state or country to adjacent or distant state or country. Such a rapid spread of infection or disease is known as Pandemic infection.^{6, 7}

Mutated viruses more often cause pandemic viral infections resulting in severe morbidity and mortality. The spread of infection can be restricted if individual suffering with viral disease is isolated during incubation and symptomatic period. Seasonal epidemics is not considered as a pandemic infection when observed same time in multiple districts, states within the country.

The flu viruses initially spread from infected animals to humans. Later the flu infection is transmitted to healthy individuals during close contact.⁶

The infection occurs only when the virus comes in contact with the mucus membrane of within mouth, nose, or eyes. 

The spread of Flu Viruses from an infected human to healthy human-

1.  In most cases, flu or influenza viruses are transmitted from an infected human to healthy individuals by direct contact like handshaking. Such contact follows touching mouth, nose, or eyes.
2. Infected individual discharges contaminated mucosal droplets containing flu viruses during cough, sneezing, or blowing the nose. The act of cough and sneezing discharges visible and microscopic droplets in the air. The inhalation or swallowing of such droplets causes the transmission of viral infection from infected to a healthy individual. 
3. The transmission of the flu virus to healthy individuals is also observed when a healthy individual touches the infected discharged that is fallen on furniture or surrounding objects.

The symptoms appear within 3 to 7 days once a virus enters the healthy human body. The period between the virus entering the human body and the appearance of symptoms is known as the incubation period.

Flu is observed every year during the fall and early summer in the USA. Mostly infection is endemic and affects children. Though the flu infection affects all age groups. Most individuals receive the yearly flu vaccine and build resistance to influenza. Few who were vaccinated occasionally suffer from mild symptoms of flu-like running nose and cough for 3 to 5 days. Those children and young individuals who do not receive vaccine suffer from moderate to severe symptoms like running nose, fever, cough, and body ache that lasts for 3 to 7 days and get better.

Most widespread seasonal flu resulting in the epidemic, as well as endemic flu, is caused by influenza virus-A and B.² The most children and adults suffer from severe upper respiratory tract symptoms and breathing difficulties when infected Influenza virus-A. Such severe disease often needs hospitalization and intensive care treatment. Influenza A is sub-type into H and N influenza virus. The flu caused by type A influenza virus mostly affects children. Influenza virus A is found in birds, pigs, and horses. Influenza virus B mainly infects humans of all ages. The symptoms of flu caused by Influenza virus-B are often milder. Symptoms observed are running nose, cough, throat irritation, and fever. Influenza virus C causes mild symptoms in all ages. Influenza virus D mostly causes infection in pigs and cattle.

Viruses are formed by a sequence of the specific number of protein molecules known as genetic selection. The mutation is caused by deletion, insertion, and rearrangement of the sequence of amino acids that forms the virus.⁸ Such viral antigenic mutation creates a colony of viruses that are resistant to the effects of the vaccine and natural human immunity. The viruses are destroyed by white blood cells, but the mutated virus resists the action of white blood cells. 

Occasionally mutation also produces viruses that are less infectious and can be used as a vaccine. Such a vaccine, when injected in human causes no symptoms but increase the secretion of antibodies. The presence of such antibodies helps white blood cells to fight and destroy viruses that are causing infection.

The viral mutation is caused by the physical effect induced by the ultraviolet light or beam of X-Ray. In a few cases, enzymes trigger the change of sequence of amino acids (protein molecules) or substitute as well as adds amino acids to the protein molecule sequence of the virus.⁸ Most influenza virus mutation is thought to be caused by natural addition or change of sequence of amino acids that form the virus.

The flu infection becomes pandemic when, within short-time several humans come in contact with flu-infected birds or animals. These individuals then carry the virus and spread it in the community. Occasionally every few years, the virulent and treatment-resistant flu virus spreads across the globe and causes pandemic disease. The virulent viruses rapidly multiply within human cells and widely spread in human organs. These viruses resist the attack by white blood cells. The human antibodies cannot neutralize the virus. Infection widely spreads from infected individuals to households, neighbors, and friends.
Similarly, some of these individuals travel for pleasure or work and spread the infection to distant cities and countries. Pandemic disease resists all treatment and causes high mortality. Hospital admission results in the spread of infection among health service providers. The mutated flu virus thus causes rapid widespread of infection that causes death in 1.2 to 10% of individuals suffering from the pandemic disease. 

Following are past influenza (flu) pandemic-⁹

1. Flu Pandemic (1889)- The flu was originated in Bukhara and Turkestan. The infection was caused by the H2N2 type A influenza virus. Over a million people died from the disease.
2. Spanish flu (1918)- Pandemic influenza caused the death of 50 to 100 million people.
3. Asian Flu (1956)- The infection was caused by the H2N2 type A influenza virus. The flu originated in China and spread through Singapore, Hong Kong, and the USA. Over 2 million people died across the world and 69,800 in the USA.
4. Hong Kong Flu (1968)- The flu was caused by an H3N2 strain of type A influenza virus. The infection was spread to other countries like the Philippines, India, Australia, Europe, and the United States. The illness caused the death of more than 1 million people.
5. H1N1 influenza virus (2009)- The pandemic influenza of 2009 was caused by the H1N1 type A influenza virus. The infection was spread in more than 206 countries. 

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