 The exact mechanisms that facilitate cross-species transmission vary by pathogen, and even for widespread diseases are often unknown; however, it is believed that viruses with very high mutation rates are able to rapidly adapt to new hosts and thereby overcome host-specific immunological defenses, allowing their continued transmission.
<clarity>  The exact mechanisms that facilitate cross-species transmission vary by pathogen, and even for widespread diseases are often unknown; however, it is believed that viruses with very high mutation rates are able to rapidly adapt to new hosts and thereby overcome host-specific immunological defenses, allowing their continued transmission.
 The exact mechanisms that facilitate cross-species transmission vary by pathogen, and even for widespread diseases are often poorly understood. It is believed that viruses with very high mutation rates are able to rapidly adapt to new hosts and thereby overcome host-specific immunological defenses, allowing their continued transmission.