Influenza pandemics have ravaged society over centuries, as each new form of influenza-A bird flu evolved. Many influenza-A pandemics have occurred at or around the time of other disease outbreaks. The first pandemics in the age of microbiology were in 1889-1880 and 1898-1900. The pandemic of 1889-1880, referred to as the Russian or Asiatic flu, came in 3 waves, and is estimated to have killed more than a million people. Scientists attempted to identify the cause of the pandemics retrospectively, and believed H2 influenza caused the 1889 pandemic; and H3 influenza (likely H3N9) caused the 1898 pandemic. Scientists also speculated it could have been a coronavirus, because the ancestor of covid-19 was found to have split from other viral species, in approximately 1890. The 1890s also had pandemics of diphtheria and typhus (bacterial); and psittacosis outbreaks connected to exotic birds sold from South America. The Spanish Flu pandemic of H1N1 influenza, started in January 1918, and came in 3 waves; and in Philadelphia, a psittacosis outbreak occurred shortly before the Spanish flu outbreak, which killed over 10,000 citizens. (The Russian flu returned in 1958; and H1N1 returned as Swine flu, in 2008, and continues to circulate in the population.)

Psittacosis provides a susceptible host for other intracellular viruses and bacteria; and co-infection can lead to RNA changes in unstable viruses. Italy has a higher rate of chronic psittacosis, due to greater contact with birds, and based on reports of many common and rare illnesses associated with psittacosis, in Italy. Brazil has been the source of outbreaks of psittacosis (atypical pneumonia), back to the 1890s, through the sale of exotic birds around the world; and pets and birds in Brazil are heavily infected with intracellular pathogens and parasites. A psittacosis outbreak was reported, in Sweden, in 2020. China and Africa are currently trying to control 4 concurrent pandemics in people, poultry and pigs—coronavirus, African Swine fever, H5N1 Bird flu, and H1N1 Swine Flu; making co-infection, spillover and the evolution of viral pathogens of greater concern.

The literature reports co-infections with psittacosis and influenza type H9N2 increases mortality; which also likely applies in covid-19, when covid-19 patients have underlying chronic intracellular infections. The difference between strains of covid-19 in China and Europe, and the severe impact of the pandemic in Italy, across Europe, and in New York City, may be the result of evolution of the coronavirus when hosted by psittacosis or other chronic intracellular pathogens, currently circulating in people and animals.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4941526/