New strains of pathogens mainly arise through mutation in their genetic material, often combined with natural selection acting on those mutations over time. In many viruses and some bacteria, additional processes like recombination, reassortment, and horizontal gene transfer also create new genetic combinations that can form new strains.

Core biological process

The central biological process is evolution by mutation and selection in the pathogen’s genome. Random copying errors during replication create new variants, and those that survive better or transmit more efficiently become established as new strains.

Key mechanisms that create new strains

  • Mutation (small genetic changes)
    • Happens whenever pathogens replicate, especially rapidly replicating RNA viruses like influenza or coronaviruses.
* Accumulated small changes in surface proteins are known as antigenic drift and can make prior immunity less effective.
  • Recombination and reassortment (genetic shuffling)
    • When two related viruses infect the same cell, they can swap pieces of genome, producing a hybrid virus with mixed traits.
* In segmented viruses like influenza, this large-scale reshuffling is called antigenic shift and can generate entirely new subtypes.
  • Horizontal gene transfer in bacteria
    • Bacteria can gain new genes (for toxins or antibiotic resistance) by picking up DNA from other microbes or the environment (transformation, plasmids, etc.).
* These acquired genes can turn a previously harmless strain into a more virulent or drug‑resistant one.

Why new strains keep appearing

  • Huge population sizes and fast replication give pathogens many chances for mutations in a short time.
  • Changing host immunity, antibiotics, vaccines, and ecological shifts all apply selection pressure, favoring variants that evade defenses or spread better.
  • This continual evolutionary arms race means new strains are an expected consequence of pathogen biology, not a rare accident.

Information gathered from public forums or data available on the internet and portrayed here.