In rural or developing regions, parasitic illnesses are more prevalent than in developed areas. These illnesses can arise in developed nations among immigrants, returning tourists, or those with weaker immune systems. Parasites often enter the body via the mouth or skin. They can be ingested in food or water contaminated with feces from an infected animal or person, or by touching objects such as knives or tools used by an infected person. The three most common parasites that affect humans are: Giardia, which is commonly found in drinking water that has been polluted with dog or cat feces; Cryptosporidium, which can be found in well water and does not require pollution for its transmission; and Toxoplasma gondii, which can be found in raw meat and vegetables that have been contaminated by cat litter. There are many other parasites that can infect humans, including Hepatitis A virus, Schistosoma mansoni, and Trichinella spiralis. Many people get sick from parasites every year. Some recover without treatment. Others suffer diarrhea, fatigue, abdominal pain, and/or fever. Infections can be serious for infants, children, pregnant women, and people with weakened immune systems. Parasitic infections can also cause blindness or mental retardation if the parasite enters the brain through the nose-mouth or lungs through the stomach. There are vaccines available to prevent some of these diseases.