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: 
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.
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.