What type of water conditions usually cause a red tide?

According to Wikipedia...

"Red tides are caused by increase in nutrients that algae need, usually due to farm runoff, causing an overpopulation. Their occurrence in some locations appear to be entirely natural, while in others they appear to be a result of human activities. The frequency and severity of algal blooms in some parts of the world have been linked to increased nutrient loading from human activities. In other areas, algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents."

As for the effect on people:

"No deaths of humans have been attributed to Florida red tide, but people may experience respiratory irritation (coughing, sneezing, and tearing) when the red tide organism (Karenia brevis) is present along a coast and winds blow its toxic aerosol onshore. Swimming is usually safe, but skin irritation and burning is possible in areas of high concentration of red tide."

I hope that answers your question.