A diatonic scale contains 3 Major and 3 minor triads derived from it's interval's, so that means of 6 commom tones shared there are 6 perfect 5th's in the scale. Example:
C Major Scale = C-D-E-F-G-A-B
Perfect 5th of start note.
C = G
D = A
E = B
F = C
G = D
A = E
B = F# (doesn't fit, 5th (F#) needs to be flattened to F (b5th) to fit)
So, the b5th of B = F (also referred as Aug 4th)
Notice that F is not the perfect 5th to any of the common tones in C Major, but has become shared with B by alteration. In order for it to share means it should work both ways;
F = C (flatten the 5th (C) to B (b5th)
So, the b5th of F = B (also referred as Dim 5th)
In the C Major scale the intervals between F (4th) and B (7th) are a Tritone which creates dissonance and instability, demanding resolution when played as a harmonic interval (notes played simultaneously).
When looking at a G7 chord, notice that the tritone is present (G-B-D-F).