system
Given the following post, respond as if you're commenting on Reddit:
human
So, where are all the beneficial infectious diseases?

As far as I can see, most infectious diseases work somewhat along these lines (obviously extremely simplified): 

* microorganism infects host
* microorganism takes energy (in var. forms) from host to reproduce
* host is none to happy about thieving microorganism & tries to get rid of them
* microorganism is happy, if expulsion gets them to a new host (therefore might actually get annoying enough, just to actively trigger expulsion, even if host's immune system would not care otherwise)

Correct so far? If so, why are there no diseases which are actually beneficial to the host - either at least beneficial enough, to offset the energy-cost, or, ideally, so beneficial, that the host will WANT to spread it to family and friends - or actively seek to get infected in the first place? E.g.: a microorganism that increases pheromone-production & thereby makes its host more attractive, would probably spread rather well.

The only cases I can think of that are at least somewhat like that are Sickle Cell Anemia, and, come to think of it, I wonder whether the species making up our gut flora do not fulfill these criteria as well (though probably not - no spreading to other hosts, apart from childbirth, so not really infectious?)...

EDIT: Yes, obviously "disease" would be the wrong term in this case, sorry for that. Make it "beneficial infections" instead, and the question still stands.
gpt
The very nature of the word disease does not allow for any kind of benefit. Anytime an organism interacts with the human body in a positive way, it is called mutualism. However, it is hypothesized that early in evolutionary history, small, bacterium-like prokaryotes developed a mutualistic relationship with eukaryotic cells and gave rise to the mitochondria, with their unique mitochondrial DNA. 

Further, there are bacteria in our intestines that also provide us with many benefits, such as breaking down things we have not digested and providing us with vitamins. Said bacteria also compete with harmful bacteria and fungus, thereby helping us to avoid infections. A woman once took antibiotics for a year and developed a yeast infection all the way into her colon because her native bacteria were being killed off, allowing the yeast to fill the empty niche. Technically we all have an "infection" of these helpful bacteria, but we don't call it a disease because its not a "dis-ease;" its a good thing.

Sickle cell anemia is not an infection. It is caused by a genetic mutation, where a glutamic acid is replaced by a valine in the protein chains of hemoglobin. It does assist in defense against malaria, but is not itself an infectious disease that is helpful. In fact, if you are homozygous for sickle cell, you will probably suffer a great deal before you die an early and painful death.

EDIT: It is also hypothesized that retroviruses (viruses that can insert their genome into ours) have made many beneficial contributions to our genome over the course of our long evolutionary history.