My MIL is traveling to paraguay. How long do you think she should wait to visit my 8 month old son once returning? There is currently the outbreak of the Zika virus there. Any input would be appreciated!
This should not be a problem at all. The person likely to suffer from Zika id the person traveling to affected areas.
Zika may cause microcephaly in fetuses, that is, while the baby is in utero. Once the baby has been born, it can't contract a birth defect per se. It can contract the illness, but not enough is known to say the illness would be any different in infants than adults (in other words, like any other run-of-the-mill viral illness, not like a dangerous one, say Hepatitis B or C.) Some people testing positive for the virus experience Guillain-Barré syndrome.
The virus is most commonly spread by mosquitoes; it can also be spread through sexual contact, but no other mode of transmission from person to person is known. Even if your mother-in-law is sick from the virus, she can't give it to your baby. If a mosquito were to bite an infected person (your MIL, say) while they are sick, then bite your infant, the virus may be transmitted in this way, but think about the odds (they are quite small.)
The illness usually lasts about a week, but is mild enough that many don't even realize they have been infected.
There is still a lot that is not known about the virus. If you need to take the absolute safest measures to be comfortable, knowing that
The incubation period (the time from exposure to symptoms) for Zika virus disease is not known, but is likely to be a few days to a week.
During the first week of infection, Zika virus can be found in the blood and passed from an infected person to a mosquito through mosquito bites
Then, if your mother-in-law is not sick on her return, a week should be time enough for a viral infection to declare itself, then another week to run it's course.