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

See also: epidemic, noxious, pestilence

• unanimity
• unanimous
• unanimously
• unannealed
• unannounced
• unanswerable
• unanswered
• unanticipated
• unapologetic
• unappareled
• unapparelled
• unapparent
• unappealable
• unappealing
• unappealingly
• unappeasable
• unappendaged
• unappetising
• unappetisingness
• unappetizing
• unappetizingness
• unappreciated
• unappreciative
• unappreciatively
• unapprehensive