Homicidal ideation caused by menopause
Pink eye (highly contagious)
Recent exposure to Tuberculosis or Measles
Projectile vomiting
Explosive diarrhea
Excessive gas
Menstrual cramps
Heavy menstrual bleeding
Hangover
Hallucinations
Migraine
Backache
Mononucleosis
Walking pneumonia
Struck by an evil spell
Mandated to work during meeting time by boss controlled by Satan
Morgellons Disease
Chronic Fatigue Syndrome
Any other invisible disease would work.