Sometimes general practioners are not enough to determine the cause of a disease. Many times new diseases occur for which there is no precedence and no set syndrome associated for which to diagnosis the illness. In instances like these it is difficult to discover the nature of the illness. However, by asking questions regarding the infectious disease cycle some clues may be readily obtained.

WHAT PATHOGEN CAUSED THE DISEASE? To answer this question Koch's postulates are modified to fit humans. (recall that Kock's postulates were developed by Robert Koch to show that Bacillus anthracis caused anthrax and Mycobacterium tuberculous caused TB. They were bsed on the premise that there was a cause and effect relationship between a microbe and a disease. These postulates are mentioned and defined in an early summary).

WHAT WAS THE SOURCE/RESERVOIR OF THE PATHOGEN? The source of a pathogen is its location immediately before occupying the host. The site or environmental local of the pathogen when the infection can occur is called the reservoir. During the period of infectivity the source is actively transmitting. Sources can be carriers, or affected individuals who can infect others.

There are different types of carriers. A carrier can be active, in which case the carrier has an overt clinical case of the disease. The carrier can be convalescent, which (if you'll recall) means that the carrier has recovered somewhat but still maintains an infectious pathogen population. A healthy carrier is one that harbors the pathogen without becoming ill. An incubatory carrier is incubating a large number of pathogens but has not gotten sick yet. A chronic carrier is one who has the pathogen for a long period of time, and finally a casual, acute, or transient carrier is one that can only transmit for a short period.

HOW WAS THE PATHOGEN TRANSMITTED? There are several methods for transferring a pathogen from a carrier to a (potential) host. Airborn transmission is defined so that any pathogen that is capable of traveling over one meter in the air and still causing infection is considered airborn transmission. Contatct transmission involves touching of some sort and is a dircet method of transmission. Vehicle transmission makes use of an inantimate source to spread the pathogen. In this case the inantimate source does not support pathogent reproduction. Finally vector transmission occurs when another animal is involved. Usually this is an arthropod (such as a tick) or a vertabrate, and there are many subcategories of vector transmission. If the pathogen does not change in the vector then the vector is simply a harborage vector. An example of a pathogen of this type is Yersina pestis, which is carried by fleas and causes the bubonic plague (black death). Biological vectors, meanwhile, feature pathogenes that undergo an essential change in the vector. An example of this is the malaria-causing Plasmodia, which cannot mature without residing in a mosquito.

WHY WAS THE HOST SUSCEPTIBLE? There are many reasons why a host may be susceptible to a particular pathogen, including immunocompromization, reduction in normal flora, or much much more, discussed previously while dealing with host responses to pathogens.

HOW DID THE PATHOGEN LEAVE THE HOST? The goal of a pathogen is to propagate, and to propagate the pathogen must exit the host. This is a problem is the host dies, so therefore most pathogens don't kill people. However, they still try to leave the scene of the crime rapidly and they do so through urine, feces, mucus, and so on. This is generally a very passive process.

Interestingly, there is correlation between virulence and modes of transmission. If a pathogen is unable to live outside the host or it must be transmitted directly it cannot totally incapacitate the host and still spread (for example STDs). But if the pathogen can easily exit and survive the outside world, then it can afford to destroy the host. Finally, if the pathogen depends on a biting vector (such as a tick) it must become a systemic infection as a bite in the arm won't pick up pathogens in the intestines.

All of the above are questions regarding pathogen transmission. To combat an epidemic only one of these links needs to be broken (generally) to break the entire chain. This is done by treating and/or quarenteening the affected, by treating sewage and chlorinating water supplies, byt destroying animal or insect reservoirs, and by setting sanitation standards for food service workers.