A new infection that simply makes people feel happy is treated as a threat by the authorities while its "victims" work to spread it to others.
| Actor | Role |
|---|---|
 George PeppardPete | Pete |
 Mary Tyler MooreLiz | Liz |
| DDom DeLuiseJ. Gardner Monroe | J. Gardner Monroe |
 John McMartinThe Mayor | The Mayor |
 Nathaniel FreyConrad | Conrad |
 Charles LaneDr. Shapiro | Dr. Shapiro |
| JJeanne ArnoldGertrude | Gertrude |
 George FurthMurgatroyd | Murgatroyd |
| MMorty GuntySgt. Gunty | Sgt. Gunty |
| JJoe PonazeckiOfficer Ponazecki | Officer Ponazecki |
 Frank CampanellaCaptain Wallace | Captain Wallace |
 Susan Saint JamesAida | Aida |
 Don StroudBarney | Barney |
| NNat PolenNot listed | Not listed |
 Thelma RitterMrs. Schwartz | Mrs. Schwartz |
 Franklin CoverMedical Expert | Medical Expert |
 Cleavon LittlePhil | Phil |
 James NobleBoard Member | Board Member |
 Paul DooleyVic | Vic |
 Salem LudwigWitter | Witter |