Mosquitoes spread a variety of diseases. When they suck blood from a human, the pathogens living inside the mosquito can travel through its sucker into the human's bloodstream. 

Malaria is the most well-known disease spread by mosquitoes. Other diseases spread by mosquitoes are also common in the tropics. Over a million people die each year from these diseases.

Mosquitoes spread diseases in the northern hemisphere as well, but these diseases are not as dangerous as tropical diseases. For example, some forms of flu can be spread by mosquitoes during the summer. 

Usually, mosquito bites are not dangerous. The spot where the mosquito has penetrated the skin can be irritated, but this can be treated with store-bought ointments and lotions.