A compound for use in preventing and / or treating RSV infection, or in preventing and / or inhibiting replication of a ribonucleic acid virus (RNA), the compound having the formula (I), formula (II), formula (III ) or formula (IV), or a pharmaceutically acceptable salt thereof, ** Formula ** in which: Ar1 is phenyl or monocyclic heteroaryl having a structure corresponding to a formula selected from the group consisting of: ** Formula ** in which R1A is selected from the group consisting of hydrogen； Not me； hydroxyl； cyano； halogen； C1-C6 alkyl； C1-C6 alkenyl； C1-C6 alkynyl； C1-C6 alkoxy； haloalkyl； haloalkoxy； oxoalkyl； alkylamino； dialkylamino； alkoxyalkyl； aminoalkyl； N-alkylaminoalkyl； N, N-dialkylaminoalkyl； -L1-C (O) -OR1 'or -L1- S (O) 2R1', in which L1 is a bond or alkylene and R1 'is hydrogen, C1-C6 alkyl or hydroxyalkyl； -L2-O-C (O) - R2 ', wherein L2 is a bond or alkylene and R2' is C1-C6 alkyl or hydroxyalkyl； -L3-C (O) -NR3'R4 ', wherein L3 is a bond or alkylene and R3' and R4 'are each independently selected from the group consisting of hydrogen, alkyl and hydroxyalkyl； -L4-NR5-C (O) -R6 ', in which L4 is a bond or alkylene, R5' is hydrogen or alkyl and R6 'is alkyl or hydroxyalkyl； sulfamoyl； N- (alkyl) sulfamoyl； N, N- (dialkyl) sulfamoyl； sulfonamide； alkylthio； and thioalkyl； R1B, R1C, R1D and R1E are each independently selected from the group consisting of hydrogen, cyano, halogen, C1-C6 alkyl, alkoxy, amino, alkylamino, dialkylamino, cycloalkyl, heterocyclyl, haloalkoxy and haloalkyl； or R1B and R1C, or R1C and R1D, or R1D and R1E taken together are -O-CH2-O-； X1A, X1B, X1C, X1D and X1E are CR1AX, CR1BX, CR1CX, CR1DX and CR1EX, respectively, or N； in which 1, 2 or 3 of X1A, X1B, X1C, X1D and X1E are N； in which R1AX is selected from the group consisting of hydrogen； Not me； hydroxyl； cyano； halogen； C1-C6 alkyl； C1-C6 alkenyl； C1-C6 alkynyl； haloalkyl； haloalkoxy； oxoalkyl； alkoxy； alkylamino； dialkylamino； alkoxyalkyl； aminoalkyl； N-alkylaminoa
