1. Exceedingly harmful
   - baneful, deadly, pernicious 
2. Likely to spread and cause an epidemic disease
   - pestilential, pestiferous, plaguey

See also: epidemic, noxious, pestilence

• help oneself
• help out
• helpdesk
• helper
• helper cell
• helper T cell
• helpful
• helpfully
• helpfulness
• helping
• helping hand
• helpless
• helplessly
• helplessness
• helpline
• helpmate
• helpmeet
• Helsingfors
• Helsinki
• helter-skelter
• helve
• helvella
• Helvella acetabulum
• Helvella crispa
• Helvella sulcata