Nagoya is in Aichi prefecture that iwhich has the second largest number of people infected people with the new coronavirus in Japan.

My wife gave me a pack of vitamin supplements before I left home so that I could build up my resistance foragainst the virus by getting them constatnly.

The word resistance implies there is something being opposed; using the word 'against' will make it clear what is opposing your resistance - the virus. You can also use the word 'to': "... I could build up my resistance [to] the virus...." The word 'to' will refer to the purpose of building your resistance. 'For' also refers to purpose, but it is used when there is some goal or someone is receiving/benefiting from your action(s).

Here are some examples:
-I bought a gift [for] my mother.
-I am getting stronger [for] boot camp.
-I completed the homework [for] my class.

I also recommend erasing the last part of the sentence. In this sentence, you use the word 'by' to explain how you are fighting the virus, but we already know how you are fighting the virus because it is the reasoning to why your wife gave you vitamins. If you did not mention the vitamins, you could say: "I am building up my resistance against the virus [by] taking vitamins."

What a kind wife in the world she is, I thought.

I would be the first person in the world who isto be killed by a family member ahead ofbefore the coronavirus if I got infected.

I hope I do not I have the virus.

Nagoya is in Aichi prefecture that iwhich has the second largest number of infected people with the new coronavirus in Japan.

My wife gave me a pack of vitamin supplements before I left home so that I could build up my resistance forto the virus by getthaving them constatntly.

What a kind wife in the world she is, I thought.

I would be the first person in the world who is killed by a family member ahinstead of the coronavirus if I got infected.

I hope not II don't have the virus. / I hope I don't get the virus

My hHope

Nagoya is in Aichi prefecture that iwhich has the second largest number of people infected people with the new coronavirus in Japan.

My wife gave me a pack of vitamin supplements before I left home so that I could build up my resistance for the virusto it by geteating them constatntly.

'It' means the coronavirus here

とのこと - is basically something you heard or were told

If you want to keep that nuance, then I would say something like:

I was told that if you constantly eat vitamin supplements, then you can build up your resistance to it.

What a kind wife in the world she is, I thought.

WBut when I left home, she said, "If you give me the virus, I will kill you!"

Usually people say you shouldn't start a sentence with 'but', but I think it is acceptable.

If I got infected with the coronavirus, then I would be the first person in the world who is killed by a family member ahead of the coronavirus if I got infectedrather than the virus.

I hope not I have the virusI don't get it.