Arbitary use of THE for generalization?

[post]

Hi

This is a sentence taken from Oxford Dictionary:

 Once in the bloodstream, the bacteria adhere to the surface of the red cells. 
  

From this sentence I assume that the reason bacteria isn't used with S for plural and the verb adhere isn't in third person is that by the bacterial they mean "bacteria in general" and not a certain bacteria.

If I am correct then my question: How come it doesn't say "the red cell" instead of "the red cells" for the same reason?

[Barb_D]

Bacteria is already plural.
Bacterium is singular.