Virus Quantification Practical. - Questions about virus quantification (Jul/29/2006 )

I was just doing my virus quantification and a few questions came out and i was curious to know..

1.How do i justify pfu/ml of the virus stock when there were more than one virus infected a host cell?

2.What are the reasons we have to do a serial dilution to count the plaque??Cant we just use the original stock?

3.What are the puroses of overlying the plate with top agar??Cant we use incubate it normally like we did for bacteria colonies?

4.Is the method for quantifying lytic phage and lysogenic phage the same??why and why not?

5.What are the factors that determine the infectivity of a host cell?

So... what answers would you expect?

I suspect that the last question will be answered by any good textbook or google if you care to look.

Not an expert on lytic phages and so on, but question 2 actually answers question one... (and vice versa if you will).