ultramicroscopic
- a too small to be seen without an ultramicroscope
- 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.
- 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 .