How did it cross the species barrier?

The crossing of the species barrier typically occurs through mechanisms such as zoonotic transmission, where a pathogen jumps from one species to another, often through close contact or shared environments. Factors like habitat destruction, increased human-animal interaction, and global travel can facilitate these jumps. Additionally, genetic mutations in pathogens can enhance their ability to infect new hosts. This process has been observed in various diseases, including those caused by viruses and bacteria.