Here's why:

* Prokaryotes: Bacteria are classified as prokaryotes, meaning they lack a true nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, but it's not enclosed within a membrane.

* Eukaryotes: Other single-celled pathogens, such as parasites like protozoa, are eukaryotes. They have a true nucleus, as well as other organelles like mitochondria and Golgi apparatus.

Examples of bacterial pathogens:

* E. coli: Causes food poisoning and urinary tract infections

* Salmonella: Causes food poisoning

* Staphylococcus aureus: Causes skin infections and pneumonia

* Streptococcus pneumoniae: Causes pneumonia and ear infections

Let me know if you'd like more information about bacteria or any specific pathogen!