Short answers, pulled from the story.
Fear is an unpleasant subjective emotional state that arises in response to perceived dangers or threats, and it is typically associated with physiological and psychological changes. Psychologists have categorized its triggers in several ways, including Jeffrey Alan Gray's list of intensity, novelty, special evolutionary dangers, social-interaction stimuli, and conditioned stimuli.
The fight-or-flight response accelerates breathing and heart rate, narrows peripheral blood vessels so blood pools inward, dilates the pupils, tenses muscles to produce goosebumps, and raises blood glucose and serum calcium. Hormones such as epinephrine, norepinephrine, and cortisol drive these changes to help an organism survive by running away or fighting the danger.
The Little Albert experiment, conducted by John B. Watson in 1920, conditioned an 11-month-old boy to fear a white rat in the laboratory. The fear generalized to other white, furry objects, including a rabbit, a dog, and a Santa Claus mask with white cotton balls in the beard.
The two amygdalae, located behind the pituitary gland, are the center of most neurobiological events associated with fear. They trigger the secretion of hormones that put the body on alert and then send the memory to the medial prefrontal cortex for storage in a process called memory consolidation.
People with Urbach-Wiethe disease, a rare genetic condition that destroys both amygdalae in late childhood, are unable to experience fear. Since the disease was discovered there have been only about 400 recorded cases, and a lack of fear can lead someone into dangerous situations they would otherwise have avoided.
Cognitive behavioral therapy helps people overcome fear by repeatedly confronting it in a safe manner, and exposure therapy has helped up to 90% of people with specific phobias significantly decrease their fear over time. Systematic desensitization replaces the fear response with relaxation through conditioning, and simpler tools include journaling and building a fear ladder scored from one to ten.
Rats infected with the toxoplasmosis parasite become less fearful of cats and sometimes seek out their urine-marked areas, behavior that often leads to them being eaten, after which the parasite reproduces within the cat. There is evidence that the parasite concentrates itself in the amygdala of infected rats.