New diseases appear all the time. Some of them have been around for ages but are detected due to improved surveillance, while others appear due to shifts in host. Others are detected because they spread geographically into a region where they are either more conspicuous or just more highly reported/investigated, or because new hosts are introduced into their original range.
For example, in the last few years alone both the snappily-titled arthrogryposis hydranencephaly syndrome (caused by Schmallenberg virus, a previously unknown bunyavirus) and bovine neonatal pancytopenia (aka 'bleeding calf syndrome') have been discovered. Both were first detected in Europe, where veterinary surveillance is among the most intense in the world; Schmallenberg was detected because of syndromic surveillance in use in NL and DE. When bluetongue first emerged in Europe a slight difference in primer sets used in Switzerland detected a novel related virus that was called Toggenberg orbivirus (later classified as a new serotype of bluetongue virus). This may have been circulating in Europe undetected for many years and it's only by chance that it was detectable by the Swiss primer set (although it doesn't appear to cause clinical disease).
Incidentally, bluetongue and two other high-impact livestock diseases originating in sub-Saharan Africa (African swine fever and African horse sickness) appeared at the end of the C19th when European settlers introduced 'improved' breeds of livestock into the region; before that the pathogens were circulating subclinically in the natural hosts (warthogs for ASF virus, zebra for AHS virus and various ruminants for BT virus).