I read something about the poisoned couple consuming aquarium cleaner that contained chloroquine. So it may have not been the chlorquine that killed him and hospitalized her. Here's a few basic facts about it:

Chloroquine is not available in the US: They use hydroxychloroquine, which is supposed to better tolerated.
For malaria, the adult dose is 400-500 mg daily.
Twice that can be lethal by cardiac arrest, so DO NOT DIY with non-doctor supervised treatment.
You may not need anywhere near the malaria dose to stop the Covid-19 virus. Dose still unknown.
Quinine (as in tonic water) may be just as effective. Unknown
All of these quinines work as zinc ionophores. They let extracellular zinc into cells through a ion-specific channel. Zinc inside the cell disrupts virus replication. If you want to read the papers on this, you can Google "zinc chloroquine virus ionophore".

Nobody claims this is a magic cure: All they have claimed so far is that victims have shorter infections.
If it slows down virus replication, that would give your own immune system time to make some antibodies.

Here's an interesting quote from the NY Times:
Quote
The University of Minnesota is conducting three studies, including one on remdesivir for seriously ill patients. A second study will give hydroxychloroquine to people who have been exposed to the coronavirus, because they live in the same household as patients, to see if the drug can prevent them from becoming infected.

The third study will use an old, safe drug called losartan, normally given to treat high blood pressure, to find out whether it can prevent mild coronavirus infections from turning more serious. The drug blocks the receptor that the virus uses to get into cells, so researchers think it might stop or slow the illness.

I am a case study of N=1 (AKA anecdotal): I am a 68 year old MS patient on immunosuppression, so a pretty damned high risk. I take a lot of Vitamin D3 for the MS and I take losartan for blood pressure. I had a very mild case over the course of about 15 days. I did stop taking the immunosuppressant drug as soon as I got the infection.

One way out there theory I think might have helped is that the MS immune system modifier I take (Tecfidera) might have stopped any cytokine storm syndrome. That's what actually kills a lot of flu victims: The body's own immune cells release a lot of killer chemicals that go wild and attack host cells. One treatment is a similar drug. Totally unknown and I doubt anybody is looking at this.