The causative agents of infectious diseases can be divided into four groups.
Prions are the most recently recognized and the simplest infectious agents, consisting of a single protein molecule. They contain no nucleic acid and therefore no genetic information: their ability to propagate within a host relies on inducing the conversion of endogenous prion protein PrPc into a protease resistant isoform PrPres.
Viruses contain both protein and nucleic acid, and so carry the genetic information for their own reproduction. However, they lack the apparatus to replicate autonomously, relying instead on 'hijacking' the cellular machinery of the host. They are small (usually less than 200 nanometres in diameter) and each virus possesses only one species of nucleic acid (either RNA or DNA).
Bacteria are usually, though not always, larger than viruses. Unlike the latter they have both DNA and RNA, with the genome encoded by DNA. They are enclosed by a cell membrane, and even bacteria which have adopted an intracellular existence remain enclosed within their own cell wall. Bacteria are capable of fully autonomous reproduction, and the majority are not dependent on host cells.
Eukaryotes are the most sophisticated infectious organisms, displaying subcellular compartmentalization. Different cellular functions are restricted to specific organelles, e.g. photosynthesis takes place in the chloroplasts, DNA transcription in the nucleus and respiration in the mitochondria. Eukaryotic pathogens include unicellular protozoa, fungi (which can be unicellular or filamentous), and multicellular parasitic worms.
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