If you touch a Portuguese man of war, you will instantly suffer redness, burning, local pain, and swelling. These signs can last as long as six weeks and can lead to scars. You can also suffer abdominal cramps, fever, sweating, dizziness, nausea, vomiting, diarrhea, weakness, and fainting. Some people have breathing and heart problems after a sting. It is important to seek immediate treatment after a sting.