Dr. Oz discussed the film Contagion with the filmmakers. One thing they discussed is how a virus can spread quickly and thoroughly amongst a population. Before that, the MythBusters did an experiment to visualize how much a virus can spread at a simple dinner party and how to minimize the spread. The experiment used a clear liquid that could be seen under an ultra-violet light.
This is an obvious technique; to use a paint or dye of some sort to see who and what is “infected” after some time. Now one might think that the spread is not as bad as virus movies would make it seem because “patient zero” may have a large amount of the virus in them, but each time they contaminate someone, they pass off less and less. As well, each contaminated person is naturally less contaminated than the person that infected them, and so passes on less and less of the virus such that at some point, only minuscule amounts are being transferred.
Imagine one person holding an unsliced loaf of bread, and each time they pass someone, they rip off a chunk and give it to them, and those people do the same. At some point, people will be passing on mere crumbs.
This seems somewhat of a relief to know that the amounts drop off, and if you get infected later on (eg, by trying to stay clean as long as possible), you would be getting little enough to be able to fight it off. Unfortunately, while this is indeed the case for things like paint or bread, it is not true for bacteria and viruses because infections reproduce. You may only receive a small amount of a virus or bacteria, however they will reproduce inside you and increase their numbers, so that when you pass it off to someone else, you give them about as large a dose as anybody else, not less. Unfortunately, attrition does not apply to viral or bacterial spread.