[colarris (OP)]

At what point in the evolution of life on Earth did diseases and viruses start to appear?

[evan_au]

The origins of life on Earth are the subject of much speculation, but any actual evidence of early organic life has basically been eaten by subsequent life forms.

I suspect that early lifeforms would have competed for access to organic molecules and necessary minerals.

It would not have been too long before they found that other lifeforms were a ready supply of these vital ingredients, and then the arms race began.

Today we see only one "Genetic code" on Earth (with some minor exceptions where different organisms sometimes interpret some codons differently).

For a virus to hijack another organism's protein production, it would need to contain instructions written in the genetic code of the target organism. This is one small step towards global domination by a single DNA code.

[SquarishTriangle]

I like to think that all life began with viruses and everything else are just vehicles for viruses to multiply and spread.

[RD]

At what point in the evolution of life on Earth did diseases and viruses start to appear?

Genetic disease will be from day-one : deleterious genetic mutations will have existed as long as life has existed.

I like to think that all life began with viruses ...

That's not possible: they're not capable of reproducing if they don't have "host" cells to parasitize.

[syhprum]

I think god really loves viruses and mankind was only created as a medium for them to flourish in.

[chris]

I like to think that all life began with viruses and everything else are just vehicles for viruses to multiply and spread.

I'm coming around to that idea... particularly since I had children...

[Bored chemist]

I strongly suspect that most viruses attack bacteria, rather than people.

[puppypower]

Normally when thinking of a viral cycle, we begin the story with the virus entering a host cell. Another way to tell this story, which could give insight into the earliest virus, is to begin the story at the DNA of the creator cell. The creator cell would be a cell that duplicates part of this genetic material for export. There is not enough DNA for a daughter cell. The goal is to transfer genetic information between members of a community. This basic concept is observed in bacteria.

Problems arose when the bacteria mechanism began to impact multicellular organisms. The cells in multicellular organisms all have the same DNA and therefore addition of extra DNA, to only some cells, can cause non-equilibrium states. 

[chris]

I like to think that all life began with viruses and everything else are just vehicles for viruses to multiply and spread.

Scientists have speculated that, in fact, giant viruses ("giruses") are evidence of this - for example: https://www.thenakedscientists.com/articles/science-news/giant-virus-comes-cold

The argument goes that these agents contain such a wide diversity of genes not see elsewhere in the biosphere that they (the viruses) must be the ancestral forms of life.

However, people are becoming less convinced of this argument these days...

[Kryptid]

I like to think that all life began with viruses and everything else are just vehicles for viruses to multiply and spread.

Scientists have speculated that, in fact, giant viruses ("giruses") are evidence of this - for example: https://www.thenakedscientists.com/articles/science-news/giant-virus-comes-cold

The argument goes that these agents contain such a wide diversity of genes not see elsewhere in the biosphere that they (the viruses) must be the ancestral forms of life.

However, people are becoming less convinced of this argument these days...

Or they could have those genes because they are descended from unicellular ancestors which also had them. Maybe they are "degenerated" prokaryotes. Basically, viruses are parasites and simplification is one of the evolutionary routes that parasites often take.