Final answer:
A pet mouse that suddenly seems less afraid is likely infected with Toxoplasmosis, caused by Toxoplasma gondii, a protozoan that can alter behavior.
Step-by-step explanation:
If your pet mouse suddenly seems less afraid of you, it might be infected with Toxoplasmosis, which is caused by the protozoan Toxoplasma gondii. This disease is known to alter the behavior of infected hosts. Domestic cats are the definitive hosts for T. gondii, and they can shed infectious oocysts in their feces, which can then infect other animals and humans.
In your pet mouse, the change in behavior could be indicative of this infection. While rabies is also known to cause changes in behavior, such as increased aggression and hydrophobia, toxoplasmosis is more commonly associated with the symptom of reduced fear.