Joe Biden said the president saw seniors, who have been more at risk in the pandemic, as "expendable".
There are sharp policy differences between the two candidates on Covid-19.
In Pennsylvania, Mr Trump told thousands of supporters he felt like "Superman" after his Covid treatment.
The president tested positive for the virus on 1 October, spent three nights in hospital and was cleared by doctors to return to the campaign trail at the weekend, holding his first rally in Florida on Monday.
Meeting senior voters at a community centre in Florida, the Democratic presidential candidate said: "You're expendable, you're forgettable, you're virtually nobody. That's how he sees seniors. That's how he sees you."
The "only senior Donald Trump seems to care about" is himself, he added.
Biden also criticised the president for holding "super-spreader parties with Republicans hugging each other without concern of the consequences", while senior citizens couldn't see their grandchildren.
National opinion polls show the President is chasing a large deficit among seniors, and his standing with older voters in Florida appears equally grim, with less than a month until election day.
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