Possible Medical Issues related to UFO encounters:

Radiation exposure symptoms

Temporary Blindness or Photokeratitis 

Brain injuries and changes to the brain

Symptoms of EM exposure

Psychological consequences (PTSD, anxiety, depression, etc.)  Please see Experiencer Support

Temporary Paralysis (see also: sleep paralysis)

Nose bleeds

Time loss/memory issues

Loss of consciousness

Implants

Contagion