H1N1 or most known as bird flu is categorized to type A influenza virus that attacks animals, including bird, pig, horse, and human. Aside from type A, there are 2 groups of influenza virus which are type B and C that can only attack human and doesn't cause epidemic (disease outbreaks in large scale). Type A has caused 3 epidemics in human history in just about 100 years.
The most terrible case that has ever occurred for the past 100 years was happened in 1918-1919 which is known as "Spanish flu (by H1N1 virus)." It has killed nearly 40 millions people including World War I soldiers. Studies have shown that the main cause (origin) of that pandemic (global epidemic) was birds.
The most possible scenario of virus development during pandemic is the virus has mutated during host transfer (transfer of virus from one host to another host on different species). When animals are infected by more than one virus strain, that different strain experiencing 'recombination' if their RNA molecules match one another which causes them to mix during viral assembly. Another major factor is mutation (virus is very prone to mutation) which creates new strain that is able to attack human cell. Because of that new strain existence, our immune system can't work effectively even for human that has high antibody titer (as healthy as a horse like they say). If that new virus strain experienced recombination with a virus strain that could circulate among humans, then the resulting virus could affect from one person to another person.
Different strains of type A has been categorized according to their protein structures on their surface, those protein structures are hemagglutinin (H) and neuraminidase (N). We have known 16 different types of hemagglutinin, protein which helps virus attachment to host cell, and 9 types of neuraminidase, an enzyme which role is to release new virus particle (virion) from infected cells by breaking glycosidic bond in glycoprotein and glycolipid. Water birds have been known to carry virus strains with all possible H and N combination!
H1N1 influenza symptoms are very similar to ordinary influenza which include fever, cough, throat infection, fatigue, muscle pain, runny nose, diarrhea, and vomiting. Those symptoms could last for days, weeks, or even more. Symptoms usually can be detected after 1-4 days after infection. This virus spreads through liquid that comes from nose and mouth, just like seasonal influenza virus. Pay attention to this: "H1N1 virus could live temporarily on dead objects, in the wet area, that is caused by the infected organisms and it's safe only when it's dry completely.
A research has been conducted that shows correlation between H1N1 infection and Reye's syndrome occurrence. The research tested alanine aminotransferase serum level (SGPT) and creatine phosphokinase (CPK) whereas CPK value isn't correlated to Reye disease, infection, and SGPT level. On the other hand, SGPT value has correlation with the infection although it doesn't have direct relationship with Reye disease occurrence.
One of the ways to enhanced immunity with H1N1 is through vaccine CAIV-T injection which is still being conducted in United States, the main targets are 5-49 years old people. Through a research that has been conducted, we have known that CAIV-T vaccine has >90% efficiency in suppressing virus growth for the detectable strain in our immune system and between 50 and 90% for different strains. CAIV-T vaccine could suppress the growth of type B influenza virus (Sichuan virus) although it has lower efficiency compared to type A virus. Injection with overdose CAIV-T doesn't give better result than normal dose (normal dose is given once every 3 weeks for total of 6 weeks). This statement still has to be studied even further.
Type A influenza virus has caused epidemic thanks to the H1N1 and H3N2 strains. A result of research which was conducted by Vanderbilt University has shown that people who have been infected by H1N1 strain in the last 20 years, has more effective immunity against H3N2 strain compared to normal human. By this research, we have known that H1N1 strains have lower virulence level compared to H3N2. A serology test using <4 years old baby and infants has been conducted to reach that statement. In this test, when given H1N1 virus (attenuated form), the babies don't have influenza syndrome but it was the other way around when given H3N2 virus (attenuated form).
