Issues with Update and script. Why is it not repeating?

my code doesnt work. its super annoying. whenever i hold down the left arrow key, it rotates once. the same as if i click it… why isnt it rotating once per frame for as long as im holding down the left arrow key? also just a side note it keeps rotating in intervals of 1 if I spam left arrow.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Rotate : MonoBehaviour {

    public int angle;

	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () {


        if (Input.GetKeyDown("left"))
            transform.Rotate(Vector3.forward * 100 * Time.deltaTime);





GetKeyDown works like a click. You need to use GetKey