Mocking is catching.

Ridicule tends to spread through a group: once one person makes someone or something a target of mockery, others may join in, copying the tone without considering whether the treatment is fair.

The saying warns that derision can become socially contagious in a figurative sense, not as literal illness. It may explain escalating teasing or caution against encouraging contempt merely because others are already participating.

Use this as a warning about laughter at another person's expense inviting imitation. It suits discussion of group behavior, teasing, gossip, or public scorn, especially when one joke prompts further unkind remarks. Avoid using it around someone currently being ridiculed, where it may sound dismissive.