Humans get these viruses after being bitten by an infected mosquito or tick. Usually, if a human who has a "Flavivirus" gets bitten by a healthy mosquito or tick, the human will not have enough of the virus in their blood to infect the insect. This means humans do not help continue the virus's life cycle (by infecting insects who go on to infect other people). However, this is not true for yellow fever or dengue fever.