International treaties prohibit the development and use of biological weapons. Yet concerns about these weapons have endured and are now escalating. Filippa Lentzos writes in a paper issued by the U.S. Marine Corps that a major source of the growing concern about future bioweapons threats stem from scientific and technical advances. Innovations in biotechnology are expanding the toolbox to modify genes and organisms at a staggering pace, making it easier to produce increasingly dangerous pathogens. Disease-causing organisms can now be modified to increase their virulence, expand their host range, increase their transmissibility, or enhance their resistance to therapeutic interventions. Scientific advances are also making it theoretically possible to create entirely novel biological weapons, by synthetically creating known or extinct pathogens or entirely new pathogens. Scientists could potentially enlarge the target of bioweapons from the immune system to the nervous system, genome, or microbiome, or they could weaponize ‘gene drives’ that would rapidly and cheaply spread harmful genes through animal and plant populations.