Final answer:
There is a constant evolutionary struggle between hosts and pathogens because each evolves mechanisms to outwit the other's defenses, such as Mycobacterium tuberculosis developing resistance to macrophages and HIV constantly changing its antigens.
Step-by-step explanation:
There is a constant evolutionary struggle between the host and the pathogen because the two are engaged in a dynamic battle for survival. Pathogens, including bacteria, viruses, and other microorganisms, have developed numerous ways to evade the immune response of their hosts. Hosts, in turn, evolve their immune systems to fight off infections more effectively. This interaction is sometimes described as an evolutionary arms race.
For instance, Mycobacterium tuberculosis is known to resist the digestive enzymes of macrophages, allowing it to persist in the host and cause chronic disease such as tuberculosis. In another example, HIV employs antigenic variation which is a strategy of rapidly changing non-essential antigens to avoid being targeted by the adaptive immune system of the host. Moreover, pathogens like Streptococcus pneumoniae develop protective capsules to prevent phagocytosis, whereas others such as Staphylococcus aureus produce toxins like leukocidin to kill phagocytes. These evolutionary tactics highlight the sophistication and adaptability of pathogens, explaining the ongoing competition for survival between them and their hosts.