1. A pathogen is a biological agent that causes disease to its host. e.g. bacteria, virus, etc.
2. Bacteriophages are virus that infect bacterial cells, e.g. T4 bacteriophage.
3. Preparation of antigenic proteins of pathogens (weakened or killed) which on inoculation into a healthy person provides temporary/permanent immunity against a particular disease.
4. Viral particles which contain only protein. They do not contain nucleic acid.
