Origin of Varroa
The original host of the Varroa mite is the Asian cousin of our bee, Apis cerana. Cerana and Mellifera are so closely related that they even mate sometimes, but the offspring eggs only develop very little before they die. It is discussed which variety is the more original than the other, cerana or mellifera.
In the 19th century our western honeybee, Mellifera, was taken to different places in Asia where the eastern honeybee, Cerana, lived, Japan, Primorski, India, etc. Transports continued. More bees were brought to Primorski area and bees were brought back closer to Europe from there. Cerana was brought to western countries for research.
In the middle of the 20th century, about 100 year after the initial transportation of Mellifera bees to Asia, these bees began to develop big problems from the mites, in Asia, and in Europe. 1968 a Cerana colony was brought to Germany by a scientist and in 1973 the first colonies died there because of varroa. But the mites were close by in the Balkan countries anyway, Primorski types of mites. And soon they had reached Finland too.
Why did it take almost 100 years to create real problems in our type of bees? Whatever reason, it was probably more than one, as is often the case. Of course mites were transferred to Mellifera right away when they met Cerana. Of course mellifera bees sometimes robbed cerana colonies and vice versa.