Location, Location, Location
The location of the symbiote within the host is important because the ability of the symbiote to influence the host, and to benefit from the host, is tied to the function that is associated with the location in the host. In lichen, the algae are contained internally in hardened and translucent chambers near the surface of the organism. The algae receive their benefit, protection, by the hardened chamber grown by the host fungus. To support the production of food, the chamber is translucent to let in sunlight, which the algae uses to produce sugars. The algae are penetrated by threads from the fungus that allow the host to remove the excess sugar produced for its own use.
Not any algae will thus unite with a fungus to form lichen: it must produce excess sugar by which it can nourish itself and its host. It also must be durable enough to endure the required penetration of the threads the fungus needs to tap into the excess sugar supply: if the fungus cannot be fed from the algae, the fungus will die, followed by that of the algae.
And not any fungus will support algae to form lichen: It must be able to grow the necessary chambers within which the algae will reside, provide water to sustain itself and the algae, build the chambers in a location where light can penetrate to the algae within, and make walls that are strong enough to protect the algae while being translucent enough to let sufficient light through them to feed the algae within. Some fungi have two forms, one when only fungus, and another when it is lichen. Doubtless, the form is different because of the specialized requirements that must be met to host the algae. (And since the form of fungus is geneticfally determined, it is interesting to ask how the DNA of fungus came to have the information necessary to transistion to a more cooperative form when in the presence of an organism with unrelated DNA.)
In this case, the needs of the algae dictate its location within the fungal host, and the host must change somewhat to accomodate the algae in that location. The needs of the host, in turn, dictate that the algae have certain strengths to fulfill its role and accomodate the needs of its host.
Because there are few other endosymbiotic life forms that are cooperatively beneficial, we will turn to the "dark side" of symbiosis and consider how parasites leverage their effects via their location.
Tape worms reside in the intestines because that is where the food is, and are designed to handle the stresses of that environment during digestion. Their location, the digestive system, determines the effect the tape worms have on the host: the location enables the parasite to live off the food the host consumes, depriving the host of the full energy benefit of the food it expends energy to eat. The tape worm affects the host by reducing the energy efficiency of the host, forcing the host to expend energy to obtain more food to compensate for what the parasite is taking. If the parasites continue to multiply and take more energy, the host cannot make up the deficit and dies as if from starvation.
A form of worm is heartworm in dogs, and which preferentially reside in the pulmonary artery (the artery going from the heart to the lungs). The main impact of the parasite is via partial blockage of the pulmonary artery which restricts blood flow to the lungs. Thus, it is often seen when the dog has distress during heavy exercise, since sedentary dogs can have heartworm, but do not show symptoms, because the demand for oxygenated blood is reduced. Thus, we see that the location of the parasite dictates how it affects its host, the symptoms that the host displays, and the conditions under which those symptoms appear.
Malaria is caused by a parasitic protozoan, and has a variety of secondary symptoms that make diagnosis difficult. This is because the protozoan mainly resides in the bloodstream feeding on red blood cells, and thus can be deposited anywhere in the body, causing the secondary symptoms. One serious secondary mode of attack is when the parasite breaks into the brain and nerves, causing brain and nerve damage. Although the protozoan form of the parasite can be killed, rendering the host seemingly free of the disease, it has been recently discovered that there is a larval form of the parasite that resides exclusively in the liver. The location of this larval form prevents effecting a complete cure because effective medicines that kill the larval form also destroy the liver. The parasite can remain in larval form for years before metamorphosing into its adult protozoan form and causing a recurrence of the disease. In this case, there are two locations that the parasite uses in its life cycle, one of which is essential to its survivablity in the host.
Note how, in these examples, the symbiote/parasite leverages its location to influence its host, for good or ill. Seeing how location is a critical factor in influence of symbiote/parasite upon the host, we will go to the Holy Bible to determine the location of the Holy Spirit within the Christian believer.
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