The first algorithm, acquaintance immunization, has been described by Cohen et al. [42] and it works as follows: pick a random node v 0 , and then pick a random acquaintance v 1 , i.e. a randomly picked neighboring node of v 0 . Immunize nodes that have been referred to as acquaintances at least n times until the desired immunization coverage is achieved. In the case n = 1, every acquaintance will be immunized immediately. The acquaintance strategy has been shown to identify highly connected individuals, particularly in fat-tailed networks (such as so-called scale-free networks).