There are two ways to do it:
1) Add code to the Click event handler which looks at your CheckBox.Checked property, and continues only if it is:
private void button18_Click_1(object sender, EventArgs e)
{
if (myCheckBox.Checked)
{
var pulse = new Thread(health);
Pulse.Start();
}
}
2) Disable the button by handling the CheckBox.CheckChanged event:
private void myCheckBox_CheckedChanged(object sender, EventArgs e)
{
button1.Enabled = myCheckBox.Checked;
}