The Premier League said on Tuesday that six people from three different clubs had tested positive for COVID-19 out of a sample of 748 and Watford had confirmed three positive cases.
"Ever since I got my positive result back on Tuesday, I've been scratching my head to try to work out how I might have got coronavirus," Mariappa, 33, told the Telegraph.
"It was a big surprise because I haven't really left the house, apart from some exercise and the odd walk with the kids. I've mainly just been homeschooling and keeping fit.
"My lifestyle is very quiet, so I really don't know how I got it. Like most people, we've been having more deliveries of food and things, so maybe that's one way, and my partner has been to the supermarket a few times."
Mariappa, who will have to self-isolate for seven days in line with Premier League guidelines, said he was in the best shape of his career and looking forward to returning to training.
"I've felt as fit as ever, probably fitter because I've been working really hard. I've been following my training programme and I've not felt out of breath or ill, or anything."
The Premier League hopes to restart its season in June, having been suspended since mid-March. Watford are due to return to training this week but skipper Troy Deeney has said he will not be present out of concern for his son's health.