system
Given the following post, respond as if you're commenting on Reddit:
human
Why dont more people in the US get illnesses from mosquitoes even though the same mosquito may extract blood from several hosts?

If a mosquito extracts blood from a squirrel, and then gets blood from you, and then gets blood from some other person, and so on, wouldnt their be a prevalence of some kind of disease spreading from host to host?
gpt
To spread via mosquitos, pathogens need to be pretty specialized. Mosquitos don't take all that much blood from one host, and transfer far less to the next host, so if the pathogen is just in the blood, it's very unlikely that there will even be a single pathogen particle in the transferred blood.

Vector-borne pathogens, like yellow fever or malaria or dengue, overcome this by replicating in the mosquito and often by entering the mosquito's salivary glands. Mosquitos transfer more saliva than blood, and if the pathogen has replicated there and is present in high concentrations, it's easier to spread.

But look at the challenges this presents to the pathogen. It has to be equally effective at replicating in a mammal and in an insect. In the mammal it has to spread into the blood and be there in high concentrations, and then it has to follow a completely different pattern in the mosquito. The body temperatures, the receptors, the cell types - everything is different, so the pathogen basically has to carry double sets of everything. 

So just generic pathogens don't do this. In the US, there are a handful of the specialized pathogens that are arthropod-born. West Nile virus is well known. There are several types of viral encephalitis (Venezuelan Equine Encephalitis, Western EE, Eastern EE). There are some tick-borne diseases like Lyme Disease and Colorado Tick Fever and so on.
 
Some of the other diseases used to be present in the US, but were eradicated. Malaria was eliminated by the precursor of the CDC in the 1950s. The process reduced the number of the worst types of mosquitos, the ones that spread some of the worst diseases. That's helped as well.

Currently of course there are some mosquito-born diseases that are trying to move back into the US, as climate change makes larger regions more attractive to the mosquitos. Zika was found in Florida. Chikunungya has been found spreading locally in Florida.