This animated Ted Talk provides details regarding the spread of germs.  Germs can be defined as disease causing microorganisms, including bacteria, viruses, fungi, and protozoa.  The video also discusses the concept of virulence, and how this factor can determine the severity of a disease causing germ.  Germs spread through their own mode of transmission, such as airborne transmission. 
 
The talk then gives examples of a couple different types of pathogens, and how certain factors (like virulence) can affect the severity of the pathogen.  The concepts covered in this video are complex, but this source provides a window into the world of immunology.