How can providing first aid care be hazardous for the person providing the assistance?

They are often coming in contact with bodily fluids and pathogens from a patient which could infect the person providing assistance. They also may be putting themselves in a dangerous environment or in the path of a dangerous individual who could even while injured harm them by assaulting them with or without a weapon. 

If the person providing the assistance does wear protective gear he/she can get the bacteria. The environment in which they are in can also affect the person, if they are in a dangerous environment they can both end up getting hurt.

As long as it was one lick and the dog is a bigger dog, it should be fine. watch it carefully for a few days.