I asked two of my friends how that felt. Both report a highly undesirable state. One had fever waves shooting up to 40 degrees centigrade, extreme muscle and joint pains and having difficulties to breathe. In combination those symptoms brought him down into misery for three weeks. He needed another three weeks to recover from those symptoms and had another three to four weeks of low energy, depleted of will, stance and power and extreme tiredness. He slowly recovered from this later on with no obvious remaining issues. It took him almost one year to get well. The other person I know who had it was a nurse, also a friend, and she suffered from extreme headaches but no fever. She lost her ability to smell or taste and after almost a year she still hasn't gotten full taste back but is slowly recovering that sense. Headaches were a constant pain through the course of three weeks. She was not able to work even after the pain had gone since she could not climb stairs or walk more than 50 meters, her lungs didn't provide enough oxygen. She needed 3 months to recover and be able to work 100% again. According to both it feels like hell. So I think from those first hand reports it really feels like hell when it hits you severely.