Normally, fear is something we feel when we hear a noise in the back yard while we are home alone at night or we come across a snarling dog baring its teeth.
Lately, with a serial shooter on the loose somewhere in the Washington area, mundane tasks like putting gas in our cars or loading packages into the trunk scare us, too.
But fear isn't just an emotion that sets us on edge; it's a primordial biological instinct that helps us survive. A lima-bean-sized structure buried inside the brain serves as the body's fear detector, enabling us to identify when something is wrong and set in motion a suitable response - even if that means getting out of the way.
"A certain amount of fear is appropriate and normal and even adaptive," said Dr. Mitchel A. Kling, associate professor of psychiatry and medicine at the University of Maryland School of Medicine and medical director of the Mood Disorders Program at the Baltimore VA Medical Center. "It's when the fear becomes too intense and too extreme that it can interfere with our ability to respond to these kinds of situations."
Fear is, in short, a defense mechanism to protect the body from harm - either real or perceived. The muscles tense. The palms sweat. The heart races. The blood pressure goes up. The body feels pumped full of energy.
"It's a warning system telling you that you are facing a potentially dangerous situation," said Dr. Rudolf Hoehn-Saric, director of the Anxiety Disorders Clinic at Johns Hopkins Hospital.
The so-called "hub of fear" is a small part of the brain called the amygdala. Using information collected by the five senses, the amygdala decides whether a situation is dangerous, then fires signals to other parts of the brain. This causes the release of certain hormones, including cortisol, the "stress hormone."
"It's the trigger," Dr. Joseph LeDoux, professor of neuroscience at the Center for Neural Science at New York University, said of the amygdala. "It's like the alarm center. When it determines danger, it produces those protective responses."
One of the first things that happens is the muscles tense up in preparation for mobilizing the energy it will take to either face - or flee from - the threat, which is known as the "fight or flight" response.
"That's why people 'freeze in fear,'" LeDoux said. "Freezing is an evolutionarily based response, because animals that tend to move around are more likely to be eaten than animals that tend to stay still."
The body automatically moves blood to the arms, legs and brain from other areas where it is not immediately needed, such as the skin and gastrointestinal tract. A person breathes heavier to get more oxygen to the blood. The heart pumps faster to get the blood where it's needed most as quickly as possible.
Muscles with a fresh supply of oxygen-rich blood can do more than "tired" muscles - so a person might be able to run faster from a threat than if he or she is simply running for fun.
"All of these circulatory changes, the dilation and constriction [of blood vessels] and the pumping of the heart is all part of a finely-tuned system to get the blood to the right spot," LeDoux said.
Fear, however, is not what turns the body's defense system on; it is what results from the brain turning the system on.
"Fear is an effect, not a cause," he said. "You're feeling fear, but your fear is not what makes your heart beat [faster]. The brain's detection of danger makes your heart beat, and, as a result, you feel the fear."
There are plenty of evolution-linked stories about the body's responses to danger. Blood is moved away from the skin, it is said, because that way a person or animal involved in a fight is less likely to hemorrhage to death if cut. Getting goose bumps is a way to get the hair to stand up straight, which can make both predator and prey look bigger and stronger.
While fear and anxiety often provoke similar physical responses, such as rapid breathing and a rapid heartbeat, they are not the same. Fear is usually a feeling in response to an immediate and specific threat; anxiety is more of a prolonged or chronic state of worry or distress over something that is anticipated or pending.
In the case of the sniper, a person will feel fear if he or she comes face to face with the person committing the crime. If the person is worried about being attacked, that is anxiety.
Just as fear helps animals adapt - and respond appropriately to - dangerous situations, so, too, can anxiety be beneficial.
Anxiety can make people study harder for an exam or be more aware of their surroundings, which means they might not find themselves in a dangerous situation in the first place.
"A little anxiety can be a great motivator," said Dr. Hinda Dubin, director of psychotherapy education for the psychiatry residency program at the University of Maryland. "But being at a high level [of] anxiety as if you're in complete danger all the time is actually hard on the body itself. ... "
Anxiety disorders include generalized anxiety, post-traumatic stress syndrome, obsessive compulsive disorder, panic disorder and phobias. Treatment options include antianxiety medication, psychotherapy and behavioral therapy.
Oscar Morgan, a senior consultant at the Alexandria, Va.-based National Mental Health Association and former director of mental health for the Maryland Department of Health and Mental Hygiene, described fear and anxiety as natural alarm clocks.
"Your body kind of gives you an alarm that something is going on internally that needs to be attended to," he said. "The issue becomes, do those feelings of fear and anxiety get to the point where you can't function and go about your daily life? It's one thing to be afraid to go outside of your house. It's another not to be able to."
Being more anxious is an understandable reaction in the wake of the recent shootings.
Morgan said he needed to get gas the other night on his way home but decided not to because he would have been the only one at the gas station.
"I decided, well, this can wait. I'm not going to be the only one pumping gas tonight. And I didn't, I just went home," he said. "That's healthy, and that's normal."
"There are certain ways that the fear can be very adaptive and helpful for us," Kling said. "The problem is, because of the uncertainty and unpredictability of these attacks, it really makes it difficult for people to feel totally safe."