The Wales centre-back intends to get on the weights to make sure he can go the distance with the high-flying Swans.
Williams said: "The virus wasn't nice. It got me for a few days and I lost a load of weight.
"I've got to get in the gym this week and work hard to get back up to my weight and add some strength.
"I wasn't really at it against Everton and was tired in the second half.
"If I'm honest, then, I'm not really happy with my performance. But I'll do what I can to be there for the team next week."