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.

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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.

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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.
Take a look at anglerfish. The female is this gigantic thing, and the male is super-tiny. He bites the female and gets embedded in there. In time, his body wastes away, and he's little more than a sack of sperm for the female to use when she needs eggs fertilized. 

There are plenty of symbiotic relationships like this in the wild. Just think of something the host needs and something the symbiote needs where pairing would help both achieve their separate needs.