An infection is an invasion of the body by microscopic organisms. A pathogen is any agent that can cause a disease.
Pathogens are often carried by “vectors”, from the Latin word vectus, “one who carries”. These are humans, animals, or microbes that carry and transmit a pathogen to others.
Many bacteria are good guys. Some, however, are pathogens and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, and bubonic plague. The most common fatal bacterial diseases affect the lungs, with tuberculosis alone killing about 2 million people a year, mostly in underdeveloped countries.
Examples of common human diseases caused by viruses include the common cold, influenza, chickenpox, rabies, hepatitis, herpes, Ebola, and Zika.