The most generally accepted definition of the meme, as a `unit of information residing in a brain' (Dawkins 1982), implies a meme-host duality which is the basis of many current developments in memetics, in particular the notion that the passage of such memes (or homoderivative mnemons, following Lynch 1998) from mind to mind constitutes a process that may be considered as `thought contagion'. A critique of religious belief and other non-rational systems of thought, as `mind viruses' (Dawkins 1993), has been built upon such a meme-host duality. This paper provides two objections to the `thought contagion'/`mind virus' theory: a) that the concept of a transmitted belief, as opposed to transmitted information, is highly problematic, and b) that in any case the concept of a meme-host duality is equally suspect. It is suggested that the least philosophically problematic constitution for a science of memetics would be to adopt a behaviourist stance towards memes, to restrict the use of the term to those replicating cultural phenomena which can be directly observed or measured (Benzon 1996). This would release us from the difficulties of the indefinable meme-host relationship, and also have the merit of making memetics more directly comparable to animal behavioural ecology, to the existing branch of social psychology known as social contagion theory, and to the sociological field of empirical diffusion studies.