Um... were you, perhaps, reading out of order? Because "parasite load" is first mentioned in the between-quote exposition immediately after the block quote that is introduced and starts off with

On the potential biological negatives of novelty-seeking:

Each person's lymphocytes learn to fight off the particular varieties of parasites that are common within his own local group, ...

and includes the sentence

They should develop a more proactive "psychological immune system" to avoid getting their mouths, noses, genitals, or skin anywhere close to potential sources of infection.

which would be... interesting... if interpreted through the lens of parasitic memes.