What type of organism causes bacterial infections?

Bacterial infections are specifically caused by bacteria, which are a type of single-celled microorganism. Bacteria are prokaryotic organisms, meaning they lack a nucleus and can reproduce independently. They can be beneficial to humans, such as those in the gut flora, but pathogenic strains can lead to infections.

The other options consist of different types of organisms that do not cause bacterial infections. Viruses, for example, are not considered bacteria and operate by infecting host cells to replicate. Protozoa are single-celled organisms as well, but they are distinct from bacteria and can cause different types of infections, not typically classified under bacterial infections. Lastly, multicellular organisms, such as fungi or larger parasites, are also not responsible for bacterial infections themselves.

Understanding these distinctions emphasizes the unique role of single-celled microorganisms, particularly bacteria, in health and disease.