The fundamental differences between viruses and bacteria, however, were not yet fully understood. From 1918 to 1920, the Spanish flu pandemic became the most devastating influenza pandemic and one of the deadliest pandemics in history. The pandemic, probably caused by H1N1, likely began in the USA before spreading worldwide by soldiers during and after the First World War. The initial wave in the first half of 1918 was relatively minor and resembled past flu pandemics, but the second wave later that year had a much higher mortality rate, accounting for most deaths. A third wave with lower mortality occurred in many places a few months after the second. By the end of 1920, it is estimated that about a third to half of all people in the world had been infected, with tens of millions of deaths, disproportionately young adults. During the 1918 pandemic, the respiratory route of transmission was clearly identified and influenza was shown to be caused by a "filter passer", not a bacterium, but there remained a lack of agreement about influenzas cause for another decade and research on influenza declined. After the pandemic, H1N1 circulated in humans in seasonal form up until the next pandemic.In 1931, Richard Shope published three papers identifying a virus as the cause of swine influenza, a then newly recognized disease among pigs that was first characterized during the second wave of the 1918 pandemic.
Bigeminy is a cardiac arrhythmia in which there is a single ectopic beat, or irregular heartbeat, following each regular heartbeat. Most often this is due to ectopic beats occurring so frequently that there is one after each sinus beat, or normal heartbeat. The two beats are figuratively similar to two twins (hence bi- + gemini). For example, in ventricular bigeminy, a sinus beat is shortly followed by a premature ventricular contraction (PVC), a pause, another normal beat, and then another PVC. In atrial bigeminy, the other "twin" is a premature atrial contraction (PAC). Cause
After any PVC there is a pause that can lead to the development of bigeminy. A PVC wavefront often encounters a refractory AV node that does not conduct the wavefront retrograde. Thus the atrium is not depolarized and the sinus node is not reset. Since the sinus P wave to PVC interval is less than the normal P–P interval, the interval between the PVC and the next P wave is prolonged to equal the normal time elapsed during two P–P intervals. This is called a "compensatory" pause. The pause after the PVC leads to a longer recovery time, which is associated with a higher likelihood of myocardium being in different stages of repolarization. This then allows for re-entrant circuits and sets up the ventricle for another PVC after the next sinus beat.