The Nature of Parasitism
An infectious agent, which all viruses and some bacteria are, is an entity which furthers its existence by invading and living off the resources of a living thing (their "host," as the term is used), doing damage to the living thing in the process. The most obvious way infectious agents damage their hosts is by consuming their substance -- eating the host from the inside out. However, most infectious agents wind up consuming only a minute portion of their hosts, so although this is the ultimate threat of any infection, it hardly ever gets to be a significant factor in the damage. Rather, other processes that take place during the infection (the process by which the agent invades and resides within the host) produce more serious damage as a by-product.
Breaking In and Breaking Through
One of those processes is the digestion or breakage of parts of the host by the infectious agent in its process of penetration. Ranging in size from microscopic cell membranes to larger structures that ordinarily perform a barrier function of some kind, the destruction of these separators can cause death to portions of the host body. Damage short of destruction of these barriers can weaken them and pave the way for infection by other agents.
The Host's Reaction
Host organisms larger than microscopic size usually have some mechanisms for repelling or limiting infections. However, in many cases, the defense processes of the host do more damage than the infectious agent itself does. This is especially a problem in animals (including humans) which have an immune system. Although the absence of a functioning immune system is a dangerous condition because it can cause infections to expand unchecked, in some cases an infection which would have trivial consequences for the host otherwise sets off a chain of responses which can lead to disability or death.
Other Means of Damage to Hosts
Some viruses and bacteria produce toxins -- substances which poison the host. Some infectious agents can occupy crucial positions in the host and crowd out the machinery which the host needs to function there, such as by blocking a vessel or interfering with the action of a heart valve. There are also viruses that cause the host to grow tumors -- abnormal growths such as cancers.
Tropism
In animals, viruses nearly always, and bacteria are frequently, attracted to a particular organ or type of tissue. (This attraction is called a tropism.) All of the immediate damage to the host by an infectious agent may be only to a particular organ or tissue, but because the parts of the body depend on each other's functions, ultimately the damage is to the entire host.
Damage to Inanimate Objects
Bacteria, but not viruses, can also produce substances which do damage to inanimate objects by reacting with them chemically.
Tags: infectious agent, infectious agents, destruction these, host infectious, host infectious agent, immune system