Question: Is it feasible for an ectoparasite such as a a Tick to create a symbiotic relationship with its host? The hosts feeds the tick and the tick has some sort of beneficial substance within its saliva. This could be some sort of naturally evolved anti-fungal or anti-biotic. This relationship would allow those with the tick to out compete those without the tick due to superior disease resistance.

Answer: 
**Conditionally beneficial parasite.**

A cool thing for a fiction would be some parasite which was not beneficial but which caused some symptom that the host could use in a particular circumstance.

For example, toxoplasmosis.
<https://www.nature.com/news/parasite-makes-mice-lose-fear-of-cats-permanently-1.13777>

> 
> A parasite that infects up to one-third of people around the world may
>  have the ability to permanently alter a specific brain function in
>  mice, according to a study published in PLoS ONE today1.
> 
> 
> Toxoplasma gondii is known to remove rodents’ innate fear of cats. The
>  new research shows that even months after infection, when parasites
>  are no longer detectable, the effect remains. This raises the
>  possibility that the microbe causes a permanent structural change in
>  the brain.
> 
> 
> 

Crazy bravery like this means mice get eaten by cats. But suppose you were a warrior mouse and you craved the crazy brave. Maybe toxo in the brain could do that for you.

---

A real life example: hookworms to treat autoimmunity.

<https://www.theguardian.com/lifeandstyle/2010/may/23/parasitic-hookworm-jasper-lawrence-tim-adams>

> 
> While in the field in Papua New Guinea in the late 1980s, Pritchard
>  noted that patients infected with the Necator americanus hookworm were
>  rarely subject to the whole range of autoimmune-related illnesses,
>  including hay fever and asthma. In the years since, Pritchard had
>  developed a thesis to support this observation through painstaking
>  clinical trials (which began after he infected himself with 50
>  hookworm). The thesis proved that hookworm, in small numbers, seemed
>  able to regulate inflammatory immune responses in their hosts
> 
> 
> 

Hookworms are looking out for their own interests by crippling the immune system of their host. But if the immune system of the host has gone rogue, crippling it up is helpful and apparently hookworms produce fewer side effects than pharmacologic methods accomplishing the same end.

---

So your parasite - it produces symptoms which advance its own cause. The parasite is not your friend. It cares only about itself. But can you use those symptoms to advance your cause too? I am reminded of the infected demon arm of Prince Ashitaka in Princess Mononoke. Not good in the long term, a demon arm. But very useful in the short term it turns out.
The medical use of parasites offers fascinating examples. 

A [leech](https://en.wikipedia.org/wiki/Hirudo_medicinalis#Beneficial_secretions) is a parasite in general, but for certain health problems it has an effect that potentially outweighs the harm. 

Immune suppressant [worms](https://en.wikipedia.org/wiki/Effects_of_parasitic_worms_on_the_immune_system#Positive_effects) presumably are big picture bad, but seem to give specific auto-immune and allergy sufferers less problems than they had before.

Additionally with some non-obvious conditions some strictly bad changes to an individuals potential to reproduce might be of positive value to their genes through relatives or already having offspring:

Some parasites modify the hosts reproduction to divert that energy to the parasite, if the parasite only takes most of that energy the social network of the host might come out ahead. 

[Some parasites](https://www.ncbi.nlm.nih.gov/pubmed/15255095) compete with each other, resulting in a host less disadvantaged than having either parasite alone. Similarly in cases where one of the parasites kills the other(s) it might not be the one most deteriorates to the host. If (like most species including humans) there are a lot of common parasites a super parasite that wipes out the others might be a net win even if it ends up killing the host eventually.