With the spread of COVID-19, health experts and researchers are continuously involved in finding the origin of the virus and its transmission to humans. To mention, COVID-19 is a type of coronavirus which is zoonotic. Meaning, the viruses get transmitted to humans from an animal source.
COVID-19 is believed to be similar to SARS-CoV which has originated from bats and got transmitted to civets and then humans. According to a study published on 3 February 2020, genome sequence obtained from COVID-19 patients were found to be 76.6 per cent identical to SARS-Cov. The study also shows that the whole-genome level of COVID-19 is 96 per cent identical to bat coronavirus. This gives insight that the outbreak of COVID-19 can be associated with bats. 
Why Bats Are the Source Of Many Infectious Diseases?
Bats are known to carry a variety of zoonotic pathogens. They belong to the second-largest group of mammals including around 19 families and 962 species worldwide. Bats contain more than 130 varieties of viruses out of which 60 species are considered zoonotic and highly pathogenic to humans. 
According to a study, bats are considered a reservoir of viruses due to amazing characteristics like food choices, population structure, solitary nature, seasonal migration, flying ability. Life span, hibernation, virus susceptibility and daily movement patterns. 
Bats are highly linked to previous pandemic outbreaks like Ebola, SARS and Nipah. This is because of the high contact between bats and humans for its meat or use in industrial products. The other reason is that different species of bats live in colonies or crowded together which makes the transmission of viruses easy among them. They also shed the virus in the environment through faeces, saliva or urine which get transmitted to humans easily.
Why Bats Themselves Don't Get Infected By The Viruses?
Bats have a long span life and can live up to 35 years. Their long life makes them sustainable to many viruses which infects them persistently. With increase in the number of viruses infecting them, their immune systems develop antibodies and become resistant against the viruses.. This is the reason why bats usually don't get infected with zoonotic viruses but may cause severe illnesses when those viruses get transmitted to humans.
Another reason is the bats' ability to fly. Bats are the world's only flying mammal. When they fly, their metabolic rate increases to 15-16 fold, which is double to the metabolic rate of running rodents and eight times higher than flying birds. Also, their body temperature spikes high during the flight. The strong metabolic rate and high body temperature help in the development of a much stronger immune system which prevents from getting infected with many viruses.