What biological process causes new strains of pathogen to develop?Watch
To try and keep it brief, pathogen evolution is governed by the Red-Queen Hypothesis.
Pathogens need to evolve and adapt faster than their host, whom is also undergoing adaption to prevent the pathogen infecting them. For this to work, the generation of genetic diversity within pathogen populations is key. The pathogens best adapted to survive in the host will be the 'fittest' and be selected for. These pathogens will eventually outgrow the 'less fit' wild-type - and emerge as new strains. The generation of genetic diversity is achieved by different processes:
In all Pathogens:
-Random Mutations (especially mutations at ANTIGENIC sites). As with all life, mutations provide the fuel for natural selection. Natural selection drives the selection for the 'fittest' strains. Mutations at antigenic sites may produce strains with immune-escape mutations or the ability to bind receptors in new species (cross-species transmission)
-Transformation (Uptake of naked DNA by competent Bacteria)
-Transduction (Introduction of new genes via phage-infection - these are called pathogenicity islands and often encode toxins and virulence factors)
-Conjugation (Exchange of genetic information via direct-contact e.g. a plasmid with an antibiotic resistance gene) - Horizontal Gene Transfer
-Genetic Recombination - If multiple viruses infect the same cell, genomes can undergo recombination. Seen in many viral families - e.g. coronaviruses, COVID-19 may be a recombinant virus)
-Genetic Reassortment - In viruses with segmented genomes, e.g. influenza, if multiple viruses infect the same cell then different genomic segments can be reassorted into the progeny. Influenza is an orthomyxovirus and so replicates in the nucleus of cells. This allows the segments to get 'mixed-up' when packaging the progeny virions. This is called antigenic shift and is the cause of flu pandemics. E.g. swine flu was a reassortment H1N1 with human, porcine and avian segments.
Some Eukaryotic Pathogens (e.g. Fungi)
-Sexual reproduction (combination of parental genomes form a new genome) - Although this is not as common.
This is just brief, with limited examples. Look up more examples on google of each process and you will find extra information!