C プログラムが数値以外の入力で無限ループに入るのはなぜですか?

#include <iostream>
#include <limits>

int main()
{
    // Define variables
    int num1, num2, total;
    char answer1;

    do
    {
        // User enters a number
        cout << "\nPlease enter a positive number and press Enter: ";
        while (!(cin >> num1))
        {
            cout << "Incorrect input. Please try again." << endl;
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        if (num1 < 0) cout << "The number you entered is negative. Please enter a positive number to continue." << endl;
    } while (num1 < 0);

    // Rest of the code goes here

    return 0;
}