In this passage, we really see the contempt Marcus has for "the sins" he described in passage 1 of Book 9. Rather than endure and potentially be "infected" by the company of pretentious, pompous liars, one should leave. According to Marcus, protection against such vices is more important than becoming infected by airborne viruses. All that he is attempting to say is that we become like the company we keep. If your friends are liars, boasters, seekers of fame and people who are focused on finding happiness in externals (riches, fame, avoidance of pain, pleasures, etc), then you too may become like them. Better to leave them and seek a life of virtue.
(see also Citadel p. 271)