When on a hardcore mode game I was killing cows in broad daylight. Then 5 small zombies appeared and killed me. I searched on the wiki and found them. How do I counter attack them since there so fast?
You weren't attacked by five of them, trust me. They spawn so rarely that meeting two at the same time in the wild is a rare occurrence. But a new feature of the latest update is that zombies "summon" (spawn) more zombies when hit - You started fighting one or two and they began spawning - and swarmed you.
So: First, a tower 2-tall under your feet, or hide behind some fence, or something else that will place you beyond their reach.
Then hit them HARD. Minimize the number of hits required to kill. If you punch them or use a stone sword, you're unlikely to break even, more will spawn than you kill. Use a diamond sword, preferably enchanted, or just contain them instead of killing. If you start swatting them around with whatever you're holding in your hand at the moment, you're in for a world of hurt.
These are a new feature which have been added in the latest updates. They are very hard to kill because they are fast, hard to hit and quick to attack. The only way you can counter attack them is to build some sort of fort which they can not get into.
Remember that they do not burn, are very fast and hard to hit. Armour up, fortify up, potion up and spam click!