Infectious diseases are caused by microorganisms. These are classified into five main groups: bacteria, viruses, fungi, protozoa and helminths (worms). Each group requires its own specific drugs for treatment. These specific drugs should kill the microorganisms without being harmful to the host. This is called selective toxicity.
Most antibiotics will treat infectious diseases without regard to the organism.
Extra info: Antimicrobial drugs are sometimes harmful to the host, but they normally are more harmful to the microbial agent than to the host.
Contamination with a microorganism alsways causes an infection.
Extra info: Contamination does not always cause an infection, this is called colonization.