Many people discourage Water Fasting but haven't done it properly or long enough. I don't know about Water Fasting and parasites but Water Fasting fixes almost every problem known to man IF done long enough for the condition at hand. Hardly anyone goes to about 40 days or until you get a sudden massive hunger which is your body's signal to resume eating as then you are entering starvation mode. All other fasts are just preparing for the max/40 day one.
To do a long fast one needs to prepare carefully. Many people jump in after being years on a processed food diet and wonder why they feel so sick. One needs to cautiously build up doing longer and longer fasts. Eliminate caffeine otherwise you'll probably get severe migraines because of caffeine withdrawl. Start with a weekly 24-40 hour fast drinking about 2 liters/quarts of purified/reverse osmosis/distilled room temperature water every 24 hours. Adjust as you need. Most people feel lousy because they're not drinking enough water. Drink enough water and you won't feel hungry at all as your stomach won't shrink. You will feel psychologically hungry but that passes as your mind gets used to it. After a while of weekly fasts and pure food eating which gets easier the more you fast, try a 3 day fast. If you're working Monday-Friday make your last meal Thursday's dinner. You won't feel any weaker all day Friday so you'll be fine at work and weakness won't occur until late in the 2nd or 3rd day (Sunday). You can have a VERY small amount of food (one food only is best - mono eating where you eat only what you crave that instant) late Sunday or wait til Monday morning to resume. Most people want strength to return by Monday so they have a little Sunday night which raises their blood Sugar giving them energy, then a small meal before work on Monday, then most of their strength will have resumed. After a while of more weekly fasts try a 7-10 day fast BUT, if it goes smoothly, it makes sense to just continue with the fast. If you hit a crisis point you can always stop and resume eating. This way you can extend it to the max (40 for most of us but it depends on many things, especially your fat level) and experience full fasting benefits.
So when people discourage water fasting make sure they are talking about 40 day fasts. Or they mention exactly the length of fasting they're talking about. Many people skip a meal and call it fasting or the misleading phrase "intermittent fasting". They're just skipping a meal. Most people would gasp if they heard of someone going without their precious meals for more than a day and going more than several days will have people thinking you've lost your mind. So don't tell anyone and just do it quietly if possible. Some people have continued work while fasting for 40 days. Probably they had higher levels of fat which gave them more caloric energy.
There's another interesting benefit for long fasts. Tumors. When you deplete your fat reserves your body searches for other sources of energy/calories. A tumor can be one of those sources. But you only get that benefit after all your fat has been used up at the end of your long fast. A leaner person may encounter this before 40 days and a heavier person may encounter this a little after 40 days. But its rare to be able to go much more than 40 days for anyone. If the tumor is not eradicated normal eating is resumed for several months and another 40 day fast is done later. Each fast buys you valuable time. It usually takes about a few weeks to fully regain your strength and shape. But as your body is working much more efficiently after the fast most people look noticeably better than before.
Finally, realize that its a very poorly thought out idea to ever Water Fast to lose weight. This is the most common reason for most people water fasting. If the exercise and diet doesn't bring your weight down then that same lifestyle will just pack on the pounds after the fast. Fasting is a cleanse - pure and simple. You clean everything else in your life....why not your body?