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• a  too small to be seen without an ultramicroscope

• Bacteriologists do not know whether an ultramicroscopic germ causes that disease or whether an ultramicroscopic virus (which may be a living organism or an active chemical .
• Common colds are infectious and are probably due to an ultramicroscopic germ.
• Felix d'Herelle thinks the bacteriophage is an ultramicroscopic living organism which is parasitic in bacteria and reproduces itself.