An antimicrobial composition for use in the treatment of an infection or a disease in a living human or animal caused by a microbe, wherein the microbe is deactivated or preferentially killed, the microbe being one of a virus, a bacterium, or a fungus, comprising: a carrier, such as a liquid, gel, or solid; and a plurality of metal nanoparticles having a particle size and a particle size distribution selected so as to selectively deactivate or preferentially kill one of a virus, a bacterium, or a fungus, characterized by : the antimicrobial composition containing 10 ppb to 100 ppm of the metal nanoparticles by weight of the antimicrobial composition; and the plurality of metal nanoparticles consisting of nonionic, ground state, spherical-shaped metal nanoparticles having a mean diameter wherein at least 99% of the spherical-shaped nanoparticles have a diameter within 30% of the mean diameter, such as within 20% of the mean diameter, or within 10% of the mean diameter, and wherein at least a portion of the metal nanoparticles comprise silver nanoparticles, wherein when the microbe is a virus, the metal nanoparticles have a particle size of 2 nm to 6.5 nm, such as 3 nm to 6 nm, or when the microbe is a bacterium, the metal nanoparticles have a particle size of 7 nm to 12 nm, such as 8 nm to 10 nm, or when the microbe is a fungus, the metal nanoparticles have a particle size of 10 nm to 18 nm, such as 11 nm to 16 nm, or 12 nm to 15 nm. a carrier, such as a liquid, gel, or solid; and a plurality of metal nanoparticles having a particle size and a particle size distribution selected so as to selectively deactivate or preferentially kill one of a virus, a bacterium, or a fungus, characterized by : the antimicrobial composition containing 10 ppb to 100 ppm of the metal nanoparticles by weight of the antimicrobial composition; and the plurality of metal nanoparticles consisting of nonionic, ground state, spherical-shaped metal nanoparticles having a mean diameter wherein at least 99% of the spherical-shaped nanoparticles have a diameter within 30% of the mean diameter, such as within 20% of the mean diameter, or within 10% of the mean diameter, and wherein at least a portion of the metal nanoparticles comprise silver nanoparticles, wherein the antimicrobial composition containing 10 ppb to 100 ppm of the metal nanoparticles by weight of the antimicrobial composition; and the plurality of metal nanoparticles consisting of nonionic, ground state, spherical-shaped metal nanoparticles having a mean diameter wherein at least 99% of the spherical-shaped nanoparticles have a diameter within 30% of the mean diameter, such as within 20% of the mean diameter, or within 10% of the mean diameter, and wherein at least a portion of the metal nanoparticles comprise silver nanoparticles, wherein when the microbe is a virus, the metal nanoparticles have a particle size of 2 nm to 6.5 nm, such as 3 nm to 6 nm, or when the microbe is a bacterium, the metal nanoparticles have a particle size of 7 nm to 12 nm, such as 8 nm to 10 nm, or when the microbe is a fungus, the metal nanoparticles have a particle size of 10 nm to 18 nm, such as 11 nm to 16 nm, or 12 nm to 15 nm. <EOS>