For many years, much of the human population has lived in ignorance of plant disease - yet the presence or absence of disease in a crop, or a forest, can mean the difference between economic success or disaster, and - even in the 21st century - the difference between life and death for millions of people. This primer raises the profile of plant infectious diseases, highlighting the scale of the problem, the risks to biosecurity, and the advances in science which are impacting on surveillance, diagnosis and prevention of disease. This text opens with an overview of the importance of plant disease in human history, before reviewing the structure and function of the main types of pathogens involved in plant infectious diseases (bacteria, viruses, fungi and fungi-like pathogens), focusing on unusual and interesting examples.