Anxiety Disorders

Causes, Diagnosis and Treatment from the National Institute of Mental Health. This is a great resource if you would like to know the different types of anxiety. The National Institute of Mental Health does a huge amount of research and is the authority on mental health.
The following anxiety disorders are discussed on this website:

generalized anxiety disorder (GAD)
obsessive-compulsive disorder (OCD),
panic disorder,
post-traumatic stress disorder (PTSD), and
social phobia (or social anxiety disorder).

Regarding causation, the NIMH article states:
“By learning more about how the brain creates fear and anxiety, scientists may be able to devise better treatments for anxiety disorders. For example, if specific neurotransmitters are found to play an important role in fear, drugs may be developed that will block them and decrease fear responses; if enough is learned about how the brain generates new cells throughout the lifecycle, it may be possible to stimulate the growth of new neurons in the hippocampus in people with PTSD.”