A visualization of<unk>  network analysis (SNA). In the 21st century, much of the study of<unk>  involves online networks and communities.<unk>  involves behaviour, emotions, or conditions spreading spontaneously through a group or network. The phenomenon has been discussed by<unk>  scientists since the late 19th century, although much work on the subject was based on unclear or even contradictory conceptions of what<unk>  is, so exact definitions vary. Some scholars include the unplanned spread of ideas through a population as<unk> , though others prefer to class that as memetics. Generally<unk>  is understood to be separate from the collective behaviour which results from a direct attempt to exert<unk>  influence. Two broad divisions of<unk>  are behavioural contagion and emotional contagion. The study of<unk>  has intensified in the 21st century. Much recent work involves academics from<unk>  psychology,<unk> ogy, and network science investigating online<unk>  networks. Studies in the 20th century typically focused on negative effects such as violent mob behaviour, whereas those of the 21st century, while sometimes looking at harmful effects, have often focused on relatively neutral contagion such as influence on shopping choices, and even on positive effects like the tendency for people to take action on climate change once a sufficient number of their neighbours do.
Social contagion
Airborne transmission
Host adaptation
Horizontal transmission
Vertical transmission
Vaccine shedding
Spillover infection
Intentional contagion of infection