[ConfusedHermit (OP)]

Is there such a thing as a disease or virus that targets a single species, or even a single individual organism?

I’m also curious if this could be done artificially. Such as making an illness for a specific person that only they will get and it being to a degree that their immune system wouldn’t win.

Sounds like an evil genius sort of thing to do without creating a pandemic, huh? :{o~

[CliffordK]

Many diseases target a single species.  This happens in humans, as well as a number of animals.
For example, AIDS only targets humans, and it can be quite difficult to culture it using other species.  Likewise with Malaria (which is transmitted from human to human by mosquitoes).

Some diseases are essentially normal "flora" in one disease, but pathogenic to other species.  Snake bites, for example, not only contain venom, but also a number of other organisms.  Cat Scratch disease might also be an example of flora that can be pathogenic.

Some diseases like trichinosis, I think, is a essentially a dead end when they reach humans.  So, it is infective in when in the normal host.  It will infect humans, but is no longer infective to other organisms.  Some mosquito borne illnesses are have humans as "dead end hosts" and never reach levels in the blood that are adequate to pass the disease on (but still can make the humans quite ill).

For biological control of invasive species, the "holy grail" is to find an organism that exclusively targets the species in question.  The Cinnabar moth was imported to control Tansy Ragweed in the Northwest USA.  And, it only eats Tansy, to the point where it almost, but not quite eliminates its host.  Biological controls are being evaluated for Japanese Knotweed in Britain, and elsewhere. 

Non host specific biological controls such as the cane toad imported to Australia can have disastrous consequences. 

[ConfusedHermit (OP)]

The only thing I knew about in your post was the bit about AIDS. Most of everything else was new! :{o~

You guys have given me so much to look into, thanks for the help.

@RD: That's a good way of putting it.

[RD]

Biological viruses can be like computer viruses.
The software may only run on a particular type of computer.

[CliffordK]

Bird Flu is another virus that has a specific host (although, perhaps it will attack a number of avian species).  In some cases it will infect humans, but is difficult to transmit from human to human. 

The concern, of course, is that it will someday learn to rapidly spread between humans, somewhat like the swine flu epidemic a few years ago.

A species jump of something like the flu could be devastating.  Consider the 1918 flu.

[Don_1]

There are viruses which may be specific to a single species, yet in other forms specific to other species.

Hold on to your seats, here come the chelonians again!!!

Herpes in humans is a rather irritating and annoying viral infection with no real serious consequences. Herpes in the chelonians (Tortoises, Turtles and Terrapins) is a serious life threatening viral infection. But these two forms of a virus do not cross species.

[CliffordK]

In some cases, herpes can also lead to serious consequences in humans..

In many cases, a disease may be species specific, but have close relatives in many other species.  For example the flea or louse is either species specific, or has its preferred host.  So, dog fleas and human lice are different closely related parasites.

[schneebfloob]

This is absolutely possible. Bacteriophages (literally "bacteria eating") are often highly specific in which bacteria they infect. Some, such as T4 and bacteriophage lambda, are specific to E.coli. This is a feature which has made them very useful in molecular biology and biotechnology.
There are many others too, that are specific to different species, and they'll possess adaptations to make them particularly efficient in catching their bacterial prey.

There are viruses that are specific to humans, such as HIV, but they are often closely related to other viruses that attack other species. HIV bears close similarity to SIV in other primates, particularly SIV in chimpanzees and Sooty Mangabeys.

[evan_au]

Another permutation is a disease which normally infects one host, but in which it causes no real symptoms. It's almost not worth calling this a "disease".

However, when a foreign host somehow gets infected, it will have no immunity, and the results can be lethal. An example is Hendra virus, which normally infects bats, but when horses are exposed, they can die quickly.

Footnote: Biological warfare is against international conventions. Besides that, it is likely to backfire.