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How does HIV enter the body?

VirusIt is common knowledge that unprotected sex with a HIV +ve person puts you at risk of getting infected with HIV. But have you ever wondered how the HIV virus actually gets into your body?

This article discusses the various ways the sneaky HIV virus dodges and avoids your natural defences to cause an infection. It is a war story. Your mucosal membranes are like the beaches of Omaha lined with fences, trenches, mines and guns. Big guns. But the HIV virus are many, and they are determine to breach the beach. This is how it plays out.

 The Enemy

HIV is a virus. That means it is a tiny microscopic particle. Imagine it is a really small ball. Inside this ball, is an even tinier strand of genetic material called RNA. Like a thread inside the ball.

The objective of HIV is to get this tiny bit of RNA into your T-cells. Once it reaches the T-Cells, it will combine with the DNA of the T-Cell and from there, it is unstoppable and incurable.

 The Defence

The problem for HIV is the T-Cells are located deep inside the body. To get from the outside of the body all the way to these T-cells deep inside, there are lots of hurdles it must overcome. These are the body’s defences.


  • Skin


The first few layers of skin are made of tough layers of dead cells called keratin. They might not look like much to you but to the HIV virus it looks like the Great Wall of China and then some. In short, HIV cannot get through intact skin.


  • Mucosal Membrane


However, during intercourse, HIV does not only come into contact with skin. The inner lining of the vaginal wall, the inner lining of the mouth, the tube inside the penis and deep in the rectum are not covered by skin. They are covered by Mucosa. The mucosa itself is not defenseless, it has tight junctions that prevent the HIV virus from getting through.


  • Anti-Microbial Peptides


This is the minefield of the body. They are basically proteins that sit around to kill any microbe that comes its way. Yes, even and especially viruses. HIV viruses.


  • Anti-Viral Antibodies


These are the sentries of the body. They float around looking for any and all viruses. Once it spots a virus, it either goes in for the kill or sounds the alarm so that other immune cells respond to kill or eat up the viruses.


The Battle


The skin is tough. We know that. We also know the HIV virus cannot penetrate skin. But wait. The HIV virus found in semen and vaginal fluid does not only come into contact with the skin during intercourse.


It also comes into contact with the area inside the vagina and inside the urinary tube of the penis. These wet areas are not skin. They are made of cells different from the skin and is called mucosa.


Mucosa unfortunately does not have the thick layer of dead cells to protect it like the skin.


The HIV virus can squeeze its way in between the mucosal cells into the sub-mucosal area. Imagine you have gaps in your wall defences and the enemy has squeezed its way between these gaps and now are in the compound of your fortress.


But fear not! You still have your minefields and sentries. Immediately, they start killing the HIV virus.


The virus knows that in order to survive, it has to find a place to hide. And what better place to hide than inside one of your own cells?


The problem is the HIV virus cannot go inside any cell. Each cell has a cell membrane that itself acts as a wall against HIV. But the sneaky HIV virus has one key (gp120) that can open one particular lock (CD4, CCR5) that is found only on a very few cells in the area underneath the mucosa.


Desperately the HIV virus looks for such a cell. More often than not, they are all killed before they manage to hide inside one of these cells.


But every so often, the HIV virus gets lucky and finds a cell with the correct lock. It fits its key into the lock and hides inside the cell.


The Sneak Attacks


Like every good enemy, the HIV virus knows that a full frontal assault like that will not very likely succeed and probably cause lots of casualties. So the HIV virus has come up with some tactics to catch the body unawares.


Dendritic Cells


Dendritic cells are the guardians of the mucosa. They have long arms and provide defence for the body. The problem is, they are also one of the cells that have the special lock (CD4, CCR5) that the HIV virus has the key (gp120)to. The other problem is sometimes these Dendritic cells stretch out their arms too far until they are on the mucosal surface i.e. outside the protective wall. This provides a portal for HIV to enter the cell even without engaging the rest of the defences.


Imagine the HIV virus scanning the wall of your fortress. Suddenly, it realizes you have parked one of your tanks outside the wall and coincidentally the HIV virus has the keys to your tank! They open the door and sneak inside. Then in the morning you drive the tank back into your fortress carrying the HIV virus along with it and helping it avoid your minefields and sentries.


Trojan Horse Leukocytes


Yes they are literally called Trojan Horse white blood cells.


The HIV virus is not just found in it’s naked form in the semen and vaginal fluid. The semen and vaginal fluid also contain white blood cells. HIV has found a way to hide inside these cells.


Your body does not recognize these white blood cells as dangerous. So will happily let them in and pass all your defences. Once inside, HIV is free to infect T-cells and cause an infection. See why it is called a Trojan Horse cell?


The end of the Battle


There can only be one of 2 outcomes. Either all the HIV viruses get killed by your body’s defences or it manages to reach the T-cells. Once the HIV viruses reaches a T-cell, it enters the cell and combines with the cell’s DNA. From there, is can force the cell to make billions more HIV viruses which will go on to infect many other T-cells. This forms what is known as the infection reservoir. Our current treatment has no way of killing HIV viruses hiding in this reservoir. So the infection remains incurable.


Fortunately, more often then not, all the HIV viruses get killed before they manage to reach the T-cells. This also explains why the risk of getting infected with HIV is small even if you had a direct sexual exposure to a HIV +ve person.


NB: This is a massive over-simplification of a very complex subject with great and detailed basic science studies done by many renown scientist. This article is in no way intended to simplify their work but more to put it in a way that can be more easily appreciated by the layman.

HIV Signs and Symptoms

About Dr. Tan
Dr. Tan graduated from the National University of Singapore in 2001. His residency was in the two largest public hospitals in Singapore; Tan Tock Seng Hospital and Singapore General Hospital.

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  1. hi doc
    i had intercourse with a women who is a swinger. I had condom on and it did not break as I check it at the end of the act. shoudl I be certain that there is no chance of infection as I do not know teh status of the woman (especially since she is a swinger she said)

    • Hi,
      There is no way to be certain there is no chance of infection, although using a condom will greatly reduce your risk. Please come visit us for screening if you are concerned.

  2. Hi Dr
    I had a mosquito biting me. It was clearly engorged with blood. Idk if it was my blood or if it was a previous host. When i squashed it. Blood splatted on where he was biting which he left a little open wound from him piercing into my skin. Could (IF THE BLOOD WAS INFECTED HIV/AIDS) infect me by entering the small opening wound? Thanks in advance im a bit worried secondly if it suddenly happened on the part that i have an open wound, can i be infected. thirdly is it true that One Virus Particle Is Enough To Cause HIV Disease. Thanks in advance im a bit worried

  3. is also true that One Virus Particle Is Enough To Cause HIV desease..becuase this what i am worried..what if the one virus particle get inside on that tiny whole created by mosquito.

    • As far as i know there are no reported case of HIV infected via mosquitoes. There is no risk of HIV transmission . The environment has to be favourable and transfer this way is very unlikely. You can click here to read more on HIV and how it can be transmitted.

      • Hi Dr

        thank you for the response but may you please kindly answer this question (is it also true that one virus particle is enough to cause HIV desease).


  4. Sir,
    Yesterday evening I went along with a group to a slum area with known high concentration of HIV infected people as part of a social service programme. While talking I was bitten by a mosquito quite a large one. Out of impulsive reaction, I squashed it and it left a large blood spot on my arm.
    My question is that if the mosquito is assumed to be carrying HIV infected blood( which it might have gotten from sucking blood of hiv infected person in that area), is it possible for me to get infected for HIV by exposure to that blood by squashing of mosquito on the same spot through the small hole in the skin that the mosquito would have created for sucking blood. I do not have any other injury or cuts in that area.
    I have read on net that mosquitos digest the HIV virus but it is not immediate but takes 1-2 days and moreover although no such cases have been reported but chances are 1 in a million, Statastics are alright, I mean it is one in a million if it happens to anybody else. If it happens to you it is 1 in 1.
    I am extremely stressed over this. Please explain what should I do. there is lot of information on this on net which is more on mosquito bite but no clear cut answer to the exact situation I have narrated of mosquito squashing and mosquito blood getting into blood stream via the hole created in skin for sucking blood. my other question is if if i squash a mosquito on the fresh wound. can i get infected if the blood is hiv positive.

    • As far as i know there are no reported case of HIV infected via mosquitoes. There is no risk of HIV transmission . The environment has to be favourable and transfer this way is very unlikely. You can click here to read more on HIV and how it can be transmitted.

  5. Hi dr.

    Can I check what is the time frame from the day I had sexual intercourse to check for STD and HIV? (Even if symptoms haven’t appear)

  6. Jonathan Thum

    Hi Dr,

    I have an unprotected sex one year ago.
    Within this year. I have undergo anonymous HIV test with Test kit for around 6-7 times but all test negative.
    Am I consider safe to infect HIV?

    • This is my return question to you: What reason do you have to think that all your 7 HIV tests are wrong? The quality of the test kit? The way you conducted it? I am assuming you did home tests. If you did it with a doctor you think the doctor made a mistake? Since you are still not convinced that you are free of HIV there must be a reason. I cannot fathom what it could possibly be.

  7. Get I get HIV infected by open mouth kissing with a girl? As I have gum bleed when I brush my teeth in the morning after that there is no more bleeding

  8. Dear Dr,

    In Sept 2015, I had a protected intercourse with this girl. And 15 mins before the intercourse, I scratched my middle finger and it bled. The cut was about 10mm long & 1mm wide. I washed my finger before the intercourse & it was no longer bleeding but the wound still looked like a fresh cut. During foreplay, I fingered her for about 2 mins.

    1) I’m really afraid due to my cut even though it was not bleeding anymore. Am I at risk of hiv+?

    In Feb 2016, I had a fever & fatigue that won’t go away which lasts for a whole month and also had my really worst diarrhoea for a week in that month. I’ve never felt so sick my entire life. From March until now I felt really depressed and I never had any sex due to fear of getting hiv+.

    2) Is hiv ars symptoms possible at 5 months after exposure?

    Thank you for your response Dr.

    • drtan

      You should be more worried of getting HIV from protected sex than from a cut. Anyway, why are you wasting your time and energy stressing about this? It has already been 5 months right? Just go for a test and put your worries to rest.

  9. I started to have unprotected sex with a random girl a week ago. After a few thrusts I realised the stupidity and put some protection on. I didn’t wash myself though. Am I a high risk? I have no idea if she is HIV

  10. I attended to a patient who is hiv positive.i cleaned his body with towel soap and water.i had a cut on my nail and I just heard he too had a wound.now I have a headache, cough and sore throat.went for hiv text a week after negative but the typhiod is high 160/300 can it be the cause of my symptoms or can I contact hiv after a week of expolusre

    • First of all kudos to you for caring for a HIV +ve person. Secondly, I’m sure you were wearing gloves when attending to him. This is part of universal precautions that all health professionals should abide by. Thirdly, you cannot get HIV from touching wounds. Lastly, please get your Typhoid infection treated. All the best to you.

  11. Can I get hiv from touching infected blood straight from the wound even if I dnt have cuts and have rinsed my hands with water only?

  12. Ricky Lee

    I had sex with a lady in one of the licensed brothels in geylang. With protection of course. My questions are:

    1. Is there a large percentage of people acquiring HIV from Geylang?
    2. should I undergo any tests?
    3. I had sex last nov 6, if i should take a test, when is the best time to visit your clinic.
    Thanks a lot!

    • 1. I don’t have this data. I would assume the risk is very small for people who visit the legal brothels. This is because the CSWs there get screened on a monthly basis. 2 Of course you should. As safe as you are, it is never 100% safe. 3. Dec 6 or earlier if you develop symptoms such as pain passing urine or a discharge from the penis.

  13. I read from the Internet that mutual mast is consider safe sex. How safe is it from hiv? Is it safe when semen is at the opening of the virgina? Does the hiv virus died once exposed to air?

    Thank you.

  14. Dr! I was infected by a gonorrhea 3 mos ago. And it is my first time to have infected.
    My question is.. Is there any chance I could get HIV at the same time? If ever the girl is HIV positive?
    Hoping for a great response!

    • Being infected with Gonorrhea increases the risk of being infected with HIV. So I have to say there is a chance you are infected with HIV if your partner was also infected with HIV. Since you have been infected with Gonorrhea, it only makes sense to get screened for all the other STDs including HIV.

      • My std is already been treated.. But I still dont know about having HIV.. Still waiting for the window period. Prior to that, I already have my culture and sensitivity test and blood work.. Except for the HIV.

  15. Dr just would like to had it been reported case or document through finger cut or bleeding finger touching urethra? How about slightly touching urethra?

  16. Dear Dr,
    I was cruised by a random guy yesterday. Driven by curiosity and horniness, I agree to let him suck me. In the end I cum in his mouth. But I did not suck him at all. Now I’m really worried and regretted being so casual. I am negative, his status is unknown. Am I at risk if he is a HIV carrier? I’m really worried till I cannot get into sleep. Dr please help. thanks!


    • Dr Justin Sim

      There is almost zero risk of HIV here. STDs on the other hand are very possible so you should get checked for those.

      • Dear Dr,

        Is there any documented case where ppl contracted HIV by letting a possible HIV carrier suck them? I heard that the lining of the urethra is very thin and it is possible for hiv to enter even without open wound on my penis, is it true?

        • Dr Justin Sim

          There have been no reported cases of HIV via oral sex. Antibiotics can help reduce the chance of transfer because they will help fight infections but this is not a safe way to prevent infections.

      • Also, I’m on doxycycline for acne treatment. I take two 100 mg doxycycline capsules daily, so far for 5 months already. Will this reduce the chance of getting other STDs?

        • Dr Justin Sim

          Regular antibiotic treatment is not a good way to prevent STDs. Antibiotics should only be used for treatment of confirmed cases.

  17. So dr after i have read through this article from your post.

    it seem like there are layer (skin and mucosal on penis) for prevent the virus go through to our blood stream even the CSW was have any present open wounds or some bleeding on her hands?

    1.) what i concern is the duration of the handjob was 3 to 5 minute and although she wash her hand with deltol before she perform and i was continue performed myself until i ejaculate. so is that any risk for me to getting infection from this scenario?

    sorry has trouble you guys again.
    thks in advances.

  18. Worried American

    02/27/15 at 3 am I engaged in anal sex with a male escort. I wore a condom but when I pulled out I noticed the condom was broken at the tip. I am uncircumcised and I don’t know when it broke but I was inside him for at least 5 min. No semen was exchanged. I asked him if he was neg in the begining and after he said he was and was tested a month ago. I panicked I went to emergency room and they gave me a shot in the butt for stds and 4 azrithomizin. They said no pep because they don’t know if he was neg and the meds could hurt more than help if he is neg. they also said CDC does not recommend in this situation if partner is not positive. I texted him yesterday after ER he said I’m pissing him off and he is clean. What do I do I can’t eat or drink afraid to subject family to anything so avoiding them PLEASE HELP ME!!!!!!
    I need help on what to do.

    • This question looks very familiar. Did I not answer it before? Anyway, I agree that the CDC does not recommend PEP in this setting. From now till you can do a conclusive HIV test, there is nothing much you can do. I know it is hard not to worry but really worrying will not change anything and can cause you to feel worse.

      • Worried American

        Dr. Does it really increase my chances because I am uncircumcised? Will 30 days be ok for the first test? My life is so messed up from this my anxiety level is high lost 12 pounds from not eating and drinking just locked away from friends and family. Thank you dr. For helping everyone with their questions . You are a great man God bless you.

        • Being circumcised reduces your chance of getting HIV. Because the baseline is uncircumcised men. 30 days test is conclusive if it is a combo test.

          • Worried American Update

            Dr 34 days after possible exposure I tested negative. My dr sent blood to a lab here in the states called LabCorp they ran a hiv 1 and hiv 2 test and it was neg. should I get a test again at the 90 day mark? Or is this test conclusive?

          • It depends on what test they did. If it was a combo test, you do not need to test again. If it was an antibody test, you need to repeat the test at 3 months post exposure.

        • dear american , a few years ago i somehow got engaged in a couple of sexual exposures with commercial women , most of them were native and young women , two were totally unprotected vaginal encounters, one was protective, one partially penetrative and remaining non-insertive , some of them were foreigners , months later when i got tested i was negative for hiv, i am circumsized though , iv drug users and gays are at the maximum risk , straight sex is also a cause if unprotected but you don’t get sick on one such encounter , i suggest you to change your sexual orientation , the viral load of the source and immune system of the host are two important factors , minute exposures are cut out by the immune system like anything , you should not be putting yourself at risk time and again , dr tan is the expert here but i have researched in this area

          • Hi Rash. I’m sure everyone in this forum appreciates the fact that you are sharing your personal experiences and the knowledge gained from your research. For that we are grateful. However, I have to ask you not to say things like “i suggest you to change your sexual orientation”. You are obviously entitled to your stand on the issue of homosexuality and we respect that. However, on this forum, we do not judge. We strongly believe that everyone is free to do what they want as long as it does not harm anyone else. We are just here to provide good, professional medical information. Regardless of race, language, religion or orientation. I hope you understand.

  19. Great article

    Especially for those without a medical background. Dr Tan, have you read the Canadian Consensus statement on HIV? http://www.pulsus.com/journals/abstract.jsp?jnlKy=3&atlKy=12838&isuKy=1209&isArt=t Specifically, I am interested in its finding that insertive oral sex poses no risk of HIV transmission. Do you agree with this? Do you agree overall with the findings of these Canadian doctors?

    • Yes I do. I personally completely agree that the insertive partner in a penile-oral sexual exposure is at no risk of HIV at all. I have the utmost respect for Canadian doctors. A lot of good research information comes out of there.

  20. Brilliant

    This is so well written doctor. I bet you will help to enlighten a lot of people worries and anxieties. Thumbs up man.

    Btw, if I’m from Malaysia and I plan to come to your clinic for hiv and stds testing. I have 3 questions.
    1) Do I need to bring my passport along? Or I can test anonymously for all the test? Or am I even allowed to step in for testing in the first place?
    2) I plan to come over to test everything for once and for all. When will be a good window period to come over. Is 6 months enough for all the stds and hiv to show up?
    3) Is the price as quote the same for foreigners (Malaysian) in your website? Or will it be higher than stated in the website?

    • 1. You are absolutely welcome to come for testing. You do not need to bring your passport. You will be asked to fill up a registration form for the testing of STDs. The HIV test can remain completely anonymous. 2. 3 months is much more than enough. 3. We charge the same price for everybody. There is no “foreigner surcharge”.

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