In addition to being transmitted by cat's nails (presumably during a scratch, hence the name), it can also be detected by contact with a splinter of wood or a rose thorn.
It is in fact caused by a microorganism called bartonella coming into contact with the blood, if it finds a weak immune system, it could reproduce and cause pathology.
Symptoms in humans
In humans, the most obvious symptoms are lymphadenitis, which is an inflammation of the lymph nodes with a consequent increase in the volume of the latter, a sense of malaise and fever. It is treatable with antibiotic therapy.