An infectious disease-causing agent is the pathogen (living micro-organism be it bacterial, viral or parasitic) that can cause disease to its host. The major infectious disease causing agents are bacteria (germ), virus, rickettsia (louse) and parasite.

Bacteria are single-celled (unicellular) living organisms with tiny flagella (a tail like appendage that they use to swim). They are invisible to the naked eye and can only be seen with a microscope. They generally have cell walls and may appear in one of several shapes. Bacillus (rodlike), coccus (spherical or ovoid), and spiral (corkscrew or curved). They reproduce by dividing themselves into equal cells (daughter cells). This process is called binary fission. They live in and reproduce in warm, moist environments in the body and other areas where they grow quickly, causing an infection.

Many bacterial diseases generally produce inflammation, swelling and pain from nerve irritation and fever caused by increased body temperature by the body fighting the disease. Bacterial infections can usually be treated with antibiotic. Some examples of bacterial disease are: Cholera - caused by the ingestion of contaminated water and food containing the vibrio cholerae agent by excrement of an individual with the disease. A second one is Syphilis. Syphilis spread by sexual intercourse of an infected person to the sexual partner or from an infected mother to her fetus containing the disease agent Treponema pallidium. Other bacterial diseases include; Dysentery, Leprosy, Plague and Scarlet Fever just to name a few.

Viruses unlike bacteria are smaller and can only be seen using an electron microscope. They are acellular (not cellular) and are structurally very simple. This makes viruses the smallest life form existing. A virus contains a core made of one type of nuclei acid molecule, which can either be a DNA or RNA. This contains the virus's genes. The core of a virus is usually covered by a protein coat and sometimes may be encased by an additional layer (lipid (fat) membrane) called an envelop. They can be rod-shaped, sphere-shaped, or multisided.

Viruses lack the means for self-reproduction outside a host cell and depend on their host cellular system to reproduce. Thus in a sense, they are parasite of a different life form. Unlike parasites however, they are not considered to be to be truly alive. This is due to the fact that when they are outside of a living host they are inert, and are considered living when they multiply within a host cell that they infect. The virus DNA enters cells and uses its DNA to make copy of itself, similar to what the ordinary cell would do. This means that the host body cell is tricked into making many copies of the virus inside the cell host cell thus, killing the cell where the viruses then enter (infecting) other cells to repeat the process. Inside the body viruses produce toxins (poisons) that can cause rashes, aches and fevers. A Virus is very difficult to kill and cannot be killed with antibiotics like bacteria. Some examples of viral diseases are: Aids - transmitted through bodily fluid through from an infected human to another through semen, vaginal secretion, blood and the sharing of needles by the disease agent HIV 1 and HIV 11 and Smallpox - through human to human contact through sneezing and coughing by the disease agent variola. Others include Influenza, Yellow Fever and Lassa Fever just to name a few.

Rickettsia is any group of parasitic bacteria that live in arthropods (e.g. ticks, lice, fleas and mites) and can cause disease if transmitted to humans. Thus, transmission occurs through the infected arthropod vector. Rickettsiae can only survive inside cells and is spread through the bloodstream of the host. They divide (reproduce) by a process called binary fission. Therefore, Rickettsia is some where between bacteria and viruses in comparison. Rickettsia cause disease by the damaging blood vessels in various tissues and organs. Rickettsial diseases basically fall into 4 groups and some example are: 1. Typhus: epidemic typhus, scrub typhus, murine (endemic) typhus, and Brill-Zinsser disease; 2. spotted fever-Rocky Mountain spotted fever, Eastern tick-borne rickettsioses, and rickettsialpox; 3. trench fever and 4. Q fever.

Finally, parasites are organisms whose survival depends on other organisms (the host) to feed , grow and live. They live in or on the living tissue of a host organism which may cause disease to it and generally without killing the host. There are several different sizes of parasites which can be either single-celled protozoa or multi-celled parasites (e.g. worms, flukes, and insects). The infection from a parasite are often transmitted through contact with an intermediate vector or from the result of direct contact with the parasite. Some examples of parasitic infections are : Hookworm - a worm larva that hatch from eggs containing the disease agent Ancylostoma duodenal or Necator americanus found in stool of an infected organism which penetrate the skin of the victim and continue its life cycle. A second one is Malaria. Malaria is disease caused by a protozoan parasites (of disease agents; P. falciparum, P. vivax, P. ovale, and P. malariae) which is transmitted by the bite of an infected female mosquito of the genus Anopheles. Others include Schistosomiasis and Trypanoso.