Infected with Covid 19 or Infected by Covid 19.

[kumar17]

I am not sure what you mean by that. Who is "Someone"?

I am talking in general terms for third person.

[Tarheel]

I am talking in general terms and using third person.

OK

[Tarheel]

Try:

He contracted Covid, but he has already recovered.

He recovered sooner than expected. Another example:

Bob: Did you finish your homework already?
Sally: Yep.

Sally finished her homework sooner than expected. Here's another:

Bob: Did you finish sweeping out the garage already?
Jim: Yep.

Jim finished sweeping out the garage sooner than expected.. Here's another:

Bob: Have you finished eating already?
Larry: Yep.

Larry finished eating sooner than expected.

[Tdol]

One needs to send an email to his boss explaining why he was not able to finish his work on time. Will 'I had tested positive work'?

I tested is enough. The past perfect is not a more formal form of the past simple.

[tzfujimino]

Do you mean something like "I (your "someone") couldn't finish the work in time because I had tested positive for COVID-19 (and I had been hospitalized)"?

[kumar17]

You are correct.

[tzfujimino]

You are correct.

OK.
Let's wait for native speakers to respond.
(I've got Likes, but I'm not sure if my sentences are natural.)

[Piscean]

They are fine.

[Tdol]

Do you mean something like "I (your "someone") couldn't finish the work in time because I had tested positive for COVID-19 (and I had been hospitalized)"?

These are not general terms, though- they are specific contexts.