Here is a non-exclusive list of things that have been referred to at some point as the "silent killer," according to a Google search I just did

Hypertension
Ovarian cancer
Aspartame
Diabetes
Coal
GastroIntestinal Stasis
MSG
Hepatitis B
Hepatitis C
Unsafe water
Obesity
Whitespace
Too much sitting
Obama's VAT proposal
Carbon monoxide
Nitrogen
Fluoride
Debt
Bullying
Document metadata
Deep vein thrombosis
Celiac disease
Inhalant abuse
Abdominal Aortic Aneurysm
Hybrid vehicles
Network latency
Overwork in Japan
Stress
Stigma
Pneumonia
Kidney disease
Cancer
Tylenol
Marijuana
Inflation
Duct leakage
Suicide
Silence
Electricity
Body dysmorphic disorder

CONCLUSION: Other than a man charging at you screaming and firing a machine gun, everything that could possibly harm you or affect you in any way is a silent killer.