Probably less than 1,000 SARS-CoV-2 viruses entering the body within minutes can start infection, called the “infectious dose.” If many more than that enter, then that might cause greater numbers of the viruses to multiply in the body, which might cause more severe disease in some people. If many less than that enter, the immune system can probably fight them off (Karimzadeh et al.). Spike proteins on the outside of the virus attach to a protein on the outside of some kinds of human cells, called angio-tensin converting enzyme 2 (ACE2). ACE2 normally regulates other chemicals on the cells, and maintains blood pressure. The virus also hijacks other chemicals on the outside of human cells to enter the cells, especially transmembrane protease serine 2 (TMPRSS2), which normally helps the cell take proteins apart so the cell can use the amino acid parts to make other proteins. So, SARS-CoV-2 infects cells which have ACE2 and TMPRSS2, and causes COVID-19 disease. The nose has a lot of ACE2, with the eyes, lungs, intestines, and other organs having lesser amounts. So, the infection usually starts in the nose and eyes, spreads through the respiratory system, and then to many organs (Fischetti et al.; Matheson, Lehner).
In some COVID-19 patients, SARS-CoV-2 might have entered the eyes of those who had not been wearing face shields (Lu CW, Liu, et al.). Some patients had prodromal ocular symptoms (irritation, conjunctivitis) before onset of disease in other body parts (Hong et al.). RT-PCR found SARS-CoV-2 in the tears and conjunctival secretions of some patients with dry eyes and conjunctivitis (Aiello et al.; Chen L, Liu M et al.; Chen MJ et al.; Wu P et al.; Xia et al.; Zhang X et al.), and some patients without eye symptoms (Xie et al.). SARS-CoV-2 replicates competently in conjunctiva (Hui et al.) The flow of tears to the nasolacrimal system, and the innate immune system, can prevent most pathogens from entering the eyes. But if SARS-CoV-2 gets past that, and infect some eyes cells via their ACE2 receptors (Zhou et al.), then viruses could flush with tears into the nasopharyngeal space, and then to the respiratory system (Hong N, Yu, et al.; Napoli et al.). For these reasons, COVID-19 might be transmitted from an infected person’s eye to others. So, people in general should avoid touching near their eyes, and people near COVID-19 patients should wear face shields or goggles (Chen MJ, Chang, et al.; Lu CW, Liu,et al.; Napoli et al.; Wu P et al.).
COVID-19 is probably not sexually transmitted, but might be (Entezami et al.). SARS-CoV-2 RNA was not found in vaginas of infected women, perhaps because vaginal tissue lacks angiotensin-converting enzyme 2 (ACE2), the cell receptor for SARS-CoV-2 (Cui et al.; (Entezami et al.). ACE2 is more highly expressed in testes than ovaries, so testes might serve as reservoirs for SARS-CoV-2, which might explain why more men were severely infected than women (Entezami et al.;Shastri et al.). Some studies found SARS-CoV-2 RNA in semen (Li D, Jin, et al.; Shapiro), but others did not find it in semen (Pan F, Xiao, et al.) and expressed prostatic secretions (EPS) of infected men (Entezami et al.; Quan et al.). Scientists do not yet know clearly if SARS-CoV-2 could cause problems for natural reproduction or assisted reproduction technologies (ART) (Entezami et al.).
Even if COVID-19 is probably not transmitted by sexual intercourse, sexual interactions could transmit it through other routes described above (Cipriano et al.). The least risky would be abstinence, higher risk is partners living together, even higher risk is partners not living together using face masks, and highest risk is having several partners not using masks (Solé; Turban, Keuroghlian, Mayer).
When COVID-19 infected pregnant women gave birth, most infants were not infected. A few were infected briefly, however, indicating possible intrauterine vertical transmission from the mother through the placenta to the fetus (Chen H, Guo, et al.; Chen Y, Peng, et al.; Kelvin and Halperin; Qiao; Van Beusekom, COVID19; Yu N, Li et al.; Zeng et al.; Zhang Z, Yu, et al.) and the UK (Mullins et al.) The probable cause of not transmitting is that human placentas lack ACE2 receptors and transmembrane protease TMPRSS2, which SARS-CoV-2 uses to enter cells (Meštrović; Pique-Regi et al.) After the baby is born, the mother and her family could transmit viruses to the baby by the same routes as any other person.