[SciShow intro plays]
[Text: QQs: Why can mosquitoes transmit Zika, but not the flu?]

Hank: Are you like most humans? Do you hate mosquitoes? We talk about mosquitoes a lot because they're terrible. They transmit malaria, West Nile Virus, and now the Zika virus among many other pathogens. But what's so special about those diseases that we only get them from mosquito bites? Why can't we get other diseases from mosquitoes? Like if you have the flu and a mosquito bites you and then bites someone else, why don't they get the flu?

Well, the disease-causing organisms that mosquitoes transmit to humans are actually special cases. They evolved to both require an insect and a vertebrate as a part of their life cycle. In fact, this requirement is so specific that each disease can only be carried by specific genera or species of mosquito. For example, only mosquitoes of the genus Aedes can carry the Zika virus.

But let's back up a little there are two main types of mosquito-borne diseases in humans. Malaria is caused by a single-celled protozoan. Most of the others are caused by viruses known as arthropod-borne viruses or arboviruses for short. Arboviruses can infect both an arthropod - usually a mosquito or a tick - and a vertebrate - in this case, humans.

Typically, viruses latch on to host cells by sticking to proteins on them called receptors. If a cell doesn't have the right receptor, the virus can't get in. And arboviruses recognize receptors on both insect and human cells. They can infect them both. So an arbovirus can jump from mosquitoes to humans because it can use the receptors on insect cells to get from the mosquito's stomach to its salivary glands. From there, it can replicate and infect the next human.

By contrast, influenza viruses don't have this ability because they can't recognize insect cell receptors. So it's unlikely that it, or any regular old virus, could get from a mosquito's stomach to a human. The malaria parasites' process is even weirder. It can reproduce both sexually and asexually, but it needs a human and two mosquitoes to do it.

First, immature parasites enter the human body through the salivary glands of the mosquito. They mature in the liver and then hijack our red blood cells and divide. This is the asexual part of their life cycle. But some of these parasites - instead of just dividing - produce the precursors to sex cells, like the protozoan version of sperm and eggs. Those then get sucked up by another mosquito. Those proto-sex cells then mature and combine inside the second mosquito, which is the sexual reproduction part, producing new parasites to infect the next person. Which, I gotta say, is a pretty slick system, but that does not change how I feel about those protozoans or the mosquitoes they rode in on.

Thanks for watching and thanks especially to all of our patrons on Patreon who keep these answers coming. If you would like to submit questions to be answered, or get some videos a few days early, you can go to patreon.com/scishow and if you just want to keep getting smarter with us you can go to youtube.com/scishow and subscribe.

Viruses are master mutators and humans are master travelers, both of which could help Zika spread. Some experts think Zika came to Brazil around the time of the 2014 World Cup among the thousands...