This virus spreads in small liquid particles that are carried through the air when an infected person breathes, talks or sneezes.
The microscopic flecks of dead skin and hair that an animal sheds is known as dander and can trigger allergic reactions in some people.
Moulds rapidly spread by producing large numbers of microscopic spores that float on air currents and which can cause respiratory irritation.
Dust contains a number of tiny particles which can cause irritation and allergic reactions, such as pollen, smoke, bacteria, ash and dead skin.
These single-cell organisms can survive on non-living surfaces and can quickly multiply, sometimes causing serious infection and disease.