Complex sentence: In 2011 and 2012, cats in animal shelters in the United Kingdom had an outbreak of two different types of gammaretroviruses--the first being feline leukemia virus and the second was feline immunodeficiency virus (FIV).
Split into simpler sentences: In 2011 and 2012, cats in animal shelters in the United Kingdom had an outbreak of two different types of gammaretroviruses. The first being feline leukaemia virus (FeLV) and the second was feline immunodeficiency virus (FIV).