I had a pot of parsley on my window since fall. It was no problem with it and I was picking parsley all winter long. Yesterday I brought home some currant cuttings to force leaves for tea by putting cuttings in water. It was around 1 PM. Today at 8 AM I looked at my parsley - and sure it was covered with tiny baby aphids. I mean covered, tons of them. I am not sure if it is possible to get that much in such short period. Also, without ants, if aphids hatched from eggs on the cuttings, how did they get to my parsley few inches away from them with no contact other then the surface both parsley pot and vase with cuttings are placed on?I had to move my parsley on the deck, where most likely it will freeze tonight. And I guess I will have to throw away cutting, though i do not see any aphids there. I am starting my main crop starters in 1-2 weeks and can’t risk. I hate those tiny guys…
It’s so not fair, to us gardeners, but female aphids can be born pregnant.
That’s how quickly they can multiply…
A spray from the hose ( or insecticidal soap) usually handles 75-90% of them.
Scott
May be outside, yes. But not inside. Few years ago when we had a very warm spring and I had my deck door open I got infestation on pepper starters. What only I didn’t do… Washed each pepper in the shower, sprayed with soap water, wood ashes water, used some “safe” insecticides - only result I got was poor plants with burned leaves. And aphids came back in thew same amount. We struggled till transplanting time. 2 days outside - the bastards were gone. After all, they do not like peppers 
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And aphids can fly (wings) at some stages in their life cycle. So they could have flown over to your other plants, or perhaps there were already there and you missed them until now.
Washing does well, but needs to be repeated several times. And it is more effective if you hold the plants sideways when washing, so the bugs go down the drain not into the soil at the base. Soap based sprays and/or oil sprays also work well but need to be done several times to really lower levels. A combo of initial washing followed by a spray (after things have dried) is pretty effective. But still will need to be repeated every few days as needed for a while…
Neem oil, 3 applications 3 days apart. First application is to kill what is there, 3 days later eggs will hatch so spray again to kill those. Three days later is for good measure. This method has worked very well for me when I have manageable amount of plants to contend with. !000’s and 1000’s of lady bugs works when there are to many plants to do.
Aphids are born pregnant.