Phycodnaviridae is a type of virus that really likes eating plants that grow in water, like algae. They are made up of tiny things called molecules that are too small for you to see, but they work together to make the virus do what it needs to do.
These viruses are like little pirates that sneak up on their prey - the algae - and then take over their cells. Once they get inside, they make the cell do all kinds of things that the virus wants, like making more virus molecules and spreading the infection to other cells.
But don't worry, humans can't get sick from these viruses. They only feast on the plants in the water, not on us. So you can still have fun swimming and playing in the water without worrying about these little viruses.